Drop .NET Core 3.1 TFM

[Sergio0694]

This PR includes two changes:

• Remove the .NET Core 3.1 TFM from the HighPerformance package
• Switch all unit tests from .NET Core 3.1 to .NET 7

Ie. tests are now on .NET Framework 4.7.2, .NET 6 and .NET 7.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions