net10 target and drop redundant netstandard2.1 target

[erwinkramer]

Added net10 as target in the core projects. I removed netstandard2.1 as I don't see how that adds any value, it's overlapping with netstandard2.0, net8 and net10.

[jskeet]

I think the value of netstandard2.1 is if the project is consumed by any other netstandard2.1 libraries - otherwise they'll get the netstandard2.0 build, I believe, and that might incur more dependencies. I agree it's all a bit unfortunate though.

Is there a particular benefit in adding a net10.0 target, rather than just letting net10.0 projects consume net8.0?

[erwinkramer]

For .NET 10, I don't think we benefit from that right now in a technical sense, but when we do want to add specific features for .net 10, on a project level or in the code itself, then we're ready for it.

In general, it might clear up the library intention:

1. support net48 with netstandard2.0, and
2. directly support net8 and net10.

In theory that's all it has to support, right? Then, when net8 reaches End-Of-Support later this year, we can drop that one to make it clear we're just on the Microsoft-supported targets. I know there are edge cases, but they should be formally out of support by Microsoft, and thus by this library as well, at least that's my reasoning.