Remove nerdbank.gitversioning

Microsoft's long-term support for .NET Core 1.x and 2.0 [are over][LT…

…S]. Customers targeting these versions are in a really bad place due to no security patches being offered and thus there are likely few to none out there.

Similarly, such a critical mass of customers already have net472 on their machines that many apps and libraries now support it.

This means we can target just netstandard2.0 (mostly). We also target net472 because we offer an extension method on the WPF Dispatcher.

[LTS]: https://github.com/dotnet/core/blob/e2f22a7106860c0e5dc98bb36dc648a779944ad5/microsoft-support.md#net-core-releases

Add help links to analyzers where they are missing

Merge branch 'v15.6' into v15.7

Merge branch 'v15.7'

Merge branch 'v15.6' into v15.7

Merge pull request microsoft#250 from Microsoft/dev/andarno/MSBuild14Fix

Avoid MSBuild syntax that fails to parse in MSBuild 14

Merge pull request microsoft#245 from Microsoft/dev/andarno/RemoveHar…

…dCodedUIThreadRequirements

Support for member-level specifications regarding main thread affinity

Hide AdditionalFiles from receiving projects

Fixes microsoft#223

Merge pull request microsoft#218 from Microsoft/dev/andarno/fix216

Consider IAsyncServiceProvider as a free-threaded type