Add .NET 5.0 support to security-code-scan analyzer
### Proposal Our [security-code-scan analyzer](https://gitlab.com/gitlab-org/security-products/analyzers/security-code-scan) currently supports .NET 2.1 and 3.1. Now that [5.0 has been released](https://devblogs.microsoft.com/dotnet/announcing-net-5-0/), 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.
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