Given we don't want to shrink the API surface for frameworks supported by Microsoft, this PR reduces the supported frameworks to .NET Framework 4.7, .NET 6.0, .NET Standard 2.0 (because of UWP) and .NET Standard (because of Mono, Xamarin and Unity).

We also decided to no longer test .NET Standard. The biggest risk is that we use something that compiles but doesn't work in Unity, UWP and such. Testing this with other frameworks like the unsupported .NET Core isn't going to help. We could have embraced Unity, UWP and such. But that would require quite some infrastructure, not just for us, but also for our contributors.

Closes #2251

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