Upgrade target frameworks

[Joy-less]

This pull request:

• Replaces the target frameworks net45;net46;net472;net48;netstandard2.0;netstandard2.1;netcoreapp3.1;net5.0;net6.0;net7.0;net8.0 with netstandard2.0;netstandard2.1;net8.0;net9.0 for simplicity and because many of those frameworks are long out-of-support.
• Updates the test framework to the latest version of NUnit, working around its breaking changes.