Remove .NET Standard 2.1 support

**Is your feature request related to a problem? Please describe.**
I am wondering why we support .NET Standard 2.1. .NET Standard 2.1 only adds support for Xamarin, Mono and Unity. .NET Framework support is lost due to it being .NET Standard 2.1 instead of 2.0.

**Describe the solution you'd like**
Remove the support for .NET Standard and only keep supporting .NET versions.

**Describe alternatives you've considered**
Remove support for .NET standard and add support for .NET Standard 2.0 so it does support .NET Framework.

The issue names no files, tests, or entry points. Start by locating the repository's project files and target-framework declarations, then determine whether the intended result is removing .NET Standard support entirely or retaining .NET Standard 2.0; done means the supported target frameworks match the decided scope and the existing build remains valid.

Written by the indexing model from the issue text.