Update target frameworks from .NET 7.0 to .NET 8.0 and fix test dependencies

- Upgraded main library from net7.0 to net8.0 (net7.0 is out of support)
- Upgraded test project from net7.0 to net8.0
- Updated Microsoft.NET.Test.Sdk from 16.8.0 to 17.11.1 to fix testhost assembly loading issues
- Resolves test execution failures with .NET 9.0 SDK

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: thecoderok

src/Unidecode.NET.csproj

@@ -8,7 +8,7 @@
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Description>Unidecode.NET returns transliterated string</Description>
-    <TargetFrameworks>net7.0</TargetFrameworks>
+    <TargetFrameworks>net8.0</TargetFrameworks>
     <VersionPrefix>2.2.1</VersionPrefix>
     <PackageTags>text;unicode;seo</PackageTags>
     <RepositoryType>git</RepositoryType>

test/Unidecode.NET.Tests.csproj

@@ -1,9 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net7.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3">
       <PrivateAssets>all</PrivateAssets>