Drop netcore2.1 as a target

[manfred-brands]

Fixes #3980

[manfred-brands]

The macOS failure on XML files seems unrelated.

[jnm2]

It could be convenient to do #3984 at the same time since the overall number of things would remain the same.

[manfred-brands]

In light of @CharliePoole's comments, I drop this. Replaced with #3988 which add .net6.
That PR also suppresses 'out of support' warnings.

[CharliePoole]

Looking at #3988, I think you may have misunderstood me. My point was that I think you need to keep the out of support second-level test assemblies, even though you are dropping 2.1 support from the framework itself. If I'm not mistaken, dropping the actual support is a decision @rprouse has already made.

[manfred-brands]

The framework itself is targeting net45 and netstandard2.0 and doesn't need dropping anything.
I kept netcoreapp2.1 in the actual tested assembly: nunit.framework.tests which is the only assembly actually tested.
All other assemblies are referenced by this one and only need targeting the same as the framework itself.

The only place I wasn't sure was nunitlite where I left netcoreapp2.1 unless someone tells me it can go from there.
But as I'm new to nunit internals, I might have it wrong.

[CharliePoole]

OK, I'd say wait for Rob to to tell you what he wants to do. I thought that he wanted them all gone and I was only arguing for keeping the second-level tests - those that are run by your own tests - in order to be able to prove that a user with a .NET Core 2.1 test can still use NUnit. IMO it's insufficient to just state that analytically. Ya gotta test it!

All that said, I'm not a team member and it's just one person's opinion. :-)

[rprouse]

I'd like to drop support for compiling test assemblies to 2.1, so I'd like to see this reopened.

[manfred-brands]

Rebased and also removed installing netcore 2.1 from github actions.

[rprouse]