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

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.