Support package that is only compatible to netstandard2.1

[laicasaane]

PR #702 failed because the package only support netstandard2.1 and above.

System.InvalidOperationException: The package does not contain a compatible .NET assembly netstandard2.0,netstandard2.1

Unity has been supporting netstandard2.1 for a very long while, could the requirement on netstandard2.0 be alleviated?

[bdovaz]

If you want to submit a PR, I'm all for it