Update `Microsoft.CodeAnalysis`, once `net8` / `net9` are EOL'ed

[earloc]

package Microsoft.CodeAnalysis needs to be on the lowest version supported by all supported target-frameworks.

Once net8 and net9 enter end-of-life, we can finally bump this up to a more recent version.

On the way,modernize the code base...

[earloc]

Hopefully, we will get some better info aboput which package-version is compatbile with which sdk-version. https://github.com/dotnet/roslyn/issues/77091

For now, the newly integrated matrix-build should make tracking this down in the future way more easy, see #331