Remove support for .NET 6.0

[DevD4v3]

This issue will be solved when a dotnet-linux x86 for .net 8.0 is available.
There is this: https://github.com/Servarr/dotnet-linux-x86/releases/tag/v8.0.4-90 but it is not working as it should.

For more details: ikkentim/SampSharp#438 (comment)

[DevD4v3]

This issue will be resolved once and for all when SampSharp runs as a component compatible with 64-bit open.mp.
This will allow the use of dotnet-linux-x64 (Microsoft officially provides the binaries).
You can check the progress here: https://github.com/SampSharp/openmp-sampsharp-x64-poc.