Allow use of code under later versions of GPL

The license type of Arduino Lint is hereby changed from "GPL-3.0-only" to "GPL-3.0-or-later", for the reason explained here:

https://www.gnu.org/licenses/gpl-faq.html#VersionThreeOrLater

Arduino Lint's GPL 3.0 license itself provides for either type, but the "or later" exception is only available if it is
explicitly stated:

> If the Program specifies that a certain numbered version of the GNU General Public License "or any later version"
> applies to it, you have the option of following the terms and conditions either of that numbered version or of any
> later version published by the Free Software Foundation.

This was not previously done in the Arduino Lint code base, meaning that it could not be used by code licensed under some
later version of GPL.

The previous use of the "GPL-3.0-only" license type was not a conscious decision, but simply the result of copy/pasting
the license header from another project which happened to be "GPL-3.0-only". I simply copy/pasted the license from
Arduino CLI, which is also "GPL-3.0-only".

Since Arduino has complete ownership of all code subject to this change, no external permission must be obtained to make
this change.

Author: per1234

etc/schemas/arduino-boards-txt-definitions-schema.json

@@ -2,6 +2,7 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-boards-txt-definitions-schema.json",
   "title": "Shared definitions for the Arduino boards.txt schemas",
+  "$comment": "This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "definitions": {
     "propertiesObjects": {
       "menu": {

etc/schemas/arduino-boards-txt-permissive-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-boards-txt-permissive-schema.json",
   "title": "Arduino boards.txt JSON permissive schema",
   "description": "boards.txt contains the boards definitions of Arduino platforms. See: https://arduino.github.io/arduino-cli/latest/platform-specification/#boardstxt",
-  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "menu": {

etc/schemas/arduino-boards-txt-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-boards-txt-schema.json",
   "title": "Arduino boards.txt JSON schema",
   "description": "boards.txt contains the boards definitions of Arduino platforms. See: https://arduino.github.io/arduino-cli/latest/platform-specification/#boardstxt",
-  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "menu": {

etc/schemas/arduino-boards-txt-strict-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-boards-txt-strict-schema.json",
   "title": "Arduino boards.txt JSON strict schema",
   "description": "boards.txt contains the boards definitions of Arduino platforms. See: https://arduino.github.io/arduino-cli/latest/platform-specification/#boardstxt",
-  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the boards.txt format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "menu": {

etc/schemas/arduino-library-properties-definitions-schema.json

@@ -2,6 +2,7 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-library-properties-definitions-schema.json",
   "title": "Shared definitions for the Arduino library.properties schemas",
+  "$comment": "This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "definitions": {
     "general": {

etc/schemas/arduino-library-properties-permissive-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-library-properties-permissive-schema.json",
   "title": "Arduino library.properties JSON permissive schema",
   "description": "library.properties is the metadata file for Arduino libraries. This schema defines the minimum requirements for this file. See: https://arduino.github.io/arduino-cli/latest/library-specification/#library-metadata",
-  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "name": {

etc/schemas/arduino-library-properties-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-library-properties-schema.json",
   "title": "Arduino library.properties JSON schema",
   "description": "library.properties is the metadata file for Arduino libraries. See: https://arduino.github.io/arduino-cli/latest/library-specification/#library-metadata",
-  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "name": {

etc/schemas/arduino-library-properties-strict-schema.json

@@ -3,7 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-library-properties-strict-schema.json",
   "title": "Arduino library.properties strict JSON schema",
   "description": "library.properties is the metadata file for Arduino libraries. This schema defines the recommended format. See: https://arduino.github.io/arduino-cli/latest/library-specification/#library-metadata",
-  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap",
+  "$comment": "For information on the Arduino library.properties format, see https://godoc.org/github.com/arduino/go-properties-orderedmap. This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "type": "object",
   "properties": {
     "name": {

etc/schemas/arduino-package-index-definitions-schema.json

@@ -2,6 +2,7 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-package-index-definitions-schema.json",
   "title": "Shared definitions for the Arduino Package Index schemas",
+  "$comment": "This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "definitions": {
     "root": {
       "base": {

etc/schemas/arduino-package-index-permissive-schema.json

@@ -3,6 +3,7 @@
   "$id": "https://raw.githubusercontent.com/arduino/arduino-lint/main/etc/schemas/arduino-package-index-permissive-schema.json",
   "title": "Arduino Package Index JSON permissive schema",
   "description": "Package indexes define Arduino hardware packages. See: https://arduino.github.io/arduino-cli/latest/package_index_json-specification/. This schema defines the minimum accepted data format.",
+  "$comment": "This is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.",
   "allOf": [
     {
       "$ref": "arduino-package-index-definitions-schema.json#/definitions/root/permissive/object"