[chore] Drop .NET 3.1 and .NET 5.0 support

[nwithan8]

Description

• Drop support for EOL .NET 3.1 and .NET 5.0
• Remove empty directories from git

Testing

• Remove .NET 3.1 and .NET 5.0 testing suites and CI runs

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

This will require changing the "required" CI tests, as .NET 3.1 and .NET 5.0 test runs no longer exist.