Update to dotnet core 6.0 and update referenced packages

[bcbowen]

Update the target framework to .NET 6.0 and update referenced packages

[asherber]

I don't mind adding net6.0 as a target in the tests project, to prove that the library works under different runtimes, but I'm not a fan of the other suggested changes.

• Because this package works as an add-on for unit testing, I don't see a need to update dependencies. I think they should remain at the lowest level that this library works with. If we don't need newer features, there's no reason to update our dependencies, although consumers of the library are welcome to use newer versions in their testing assemblies.
• Making net6.0 the only target of the library project would be a breaking change and would make this library unusable by many people. The netstandard2.0 target can be consumed by anyone using a modern .net version (including net6.0), and the net452 target is useful for those who are still on older Framework versions.
• nbgv should probably be updated to work with current build tools, but that happens in Directory.Build.props, not in the .csproj files

I'll probably add a commit to move this from Appveyor to GitHub Actions, and I'll add the net6.0 target to the tests project and update nbgv when I do that.

[bcbowen]

That makes sense, thanks for the detailed explanation... I'll close this request