Add .NET 5.0 support to security-code-scan analyzer

Proposal

Our security-code-scan analyzer currently supports .NET 2.1 and 3.1. Now that 5.0 has been released, we should add support for it as well.

This is most likely a breaking change and this will need to be addressed either with the ability to target which version of the framework that the project/solution is built against or at least documenting this is a breaking change.