Reduce amount of target frameworks and add .NET 9 to test matrix

[Romfos]

Changes:

• Reduce amount of target frameworks: .NET 8 + .NET Standard 2.0
• Remove Microsoft.SourceLink.GitHub nuget. This is included in .NET 8 sdk, no need to use this package
• Added .NET 9 to test matrix

Motivation:

• .NET 6 EOL November 12, 2024 (https://devblogs.microsoft.com/dotnet/dotnet-6-end-of-support/)
• .NET 7 EOL May 14, 2024 (https://devblogs.microsoft.com/dotnet/dotnet-7-end-of-support/)
• .NET Standard 2.0 could be installable in .NET Framework 4.6.2+ and legacy .NET Core projects. No need to have separate tfm for older frameworks.
• Nuget package will have less size and restore is faster
• More easy for support

[Romfos]

please, help with review @JoshClose