Bump Linux runner version for build workflow

Previously, the `ubuntu-16.04` runner was used for the Linux job used to build and test the application in the "build" GitHub Actions workflow. That runner has now been removed by GitHub. In its place, we will use the runner with the oldest version of Ubuntu that is now available as a GitHub-hosted runner: `ubuntu-18.04`.

The reason for using the oldest available Ubuntu version is to provide compatibility with the widest possible range of Linux versions for the users of the Arduino Language Server.

Author: per1234

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       matrix:
         config:
-          - os: ubuntu-16.04
+          - os: ubuntu-18.04
             ExecutableSuffix: ''
             Exports: ''
           - os: macos-latest