Remove netcoreapp3.1 and net5.0 from TargetFrameworks and add net8.0

[gpetrou]

Is your feature request related to a problem? Please describe.
Currently, I see in the csproj files <TargetFrameworks>netstandard2.0;net461;netstandard2.1;netcoreapp3.1;net5.0;net6.0</TargetFrameworks>

Describe the solution you'd like
Remove netcoreapp3.1 and net5.0 and add net8.0.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[flobernd]

Thanks for hinting this. Removing the "end of support" targets is probably a good idea. net461 should be replaced by net462 as well. I have to check if we introduce any breaking change that way, but in theory netstandard2.0 and netstandard2.1 should still ensure compatibility with netcoreapp3.1 and net5 projects.