.NET Standard 2.1 for Microsoft.Extensions.TimeProvider.Testing

[neuecc]

Currently, Microsoft.Extensions.TimeProvider.Testing targets .NET Framework 4.6.2, .NET 8, and .NET 9.
Would it be possible to add .NET Standard 2.1 as a target?
The code itself seems to be fine, so it's just a matter of policy whether to add it to the target or not.
Microsoft.Bcl.TimeProvider supports .NET Standard 2.0.

To provide some context for this request, I am developing a library called R3 that depends on TimeProvider.
https://github.com/Cysharp/R3
This library is very popular for Unity game engine, and when using Microsoft.Extensions.TimeProvider.Testing in Unity,
the lack of .NET Standard 2.1 support is causing issues.

[joperezr]

@neuecc thanks for logging the issue and sorry for the delay in the reply. Yes we should do this as it will benefit library developers. Also this should be done across the board, as opposed to just for Microsoft.Extensions.TimeProvider.Testing, and I believe it is possbile to target netstandard2.0 in most places (as opposed to ns2.1) to match the pattern in the packages of dotnet/runtime.