Keep platform-dotnet wip on .NET Standard 2.0

[curtnichols]

.NET Standard 2.0 is the last Standard to be compatible with .NET Framework, so double-check that we haven't slipped ahead to 2.1 on the common platform WIP.

[curtnichols]

Counterpoints:

• Move to dotnet core 3.0 as it supports WPF on core.
• dotnet 5 is the successor to dotnet core 3.0 and .NET framework 4.x.
• It would be most maintainable to move to the latest dotnet release with the release of platform-dotnet. It won't catch anyone out as nobody uses it yet. It should not credibly constrain adoption.
• It appears F# is back in for UWP development, with better support coming in dotnet core 3.
  • https://github.com/dotnet/corert/issues/6055
  • https://github.com/microsoft/microsoft-ui-xaml/issues/740
  • https://github.com/dotnet/fsharp/issues/1096#issuecomment-301326696

Also, C# 8, with its notable safety features, requires 2.1 or 3.0.