Make UseCompatibleCmdlets not throw if default reference desktop-5.1.14393.206-windows is specified in the list of platforms and use core-6.0.2-windows as an alternative default reference

[bergmeister]

PR Summary

The UseCompatibleCmdlets rule works by flagging up cmdlets that are only either present in the default reference (desktop-5.1.14393.206-windows) or in one of the specified references. According to the code comments this is to ensure that aliases or custom functions are not flagged up as incompatible.
However, from a user's point of view, one might add the default reference also to the list of compatible references in the settings file for implicit documentation of what platforms/versions are supported and also because a user does not care how the work is done under the covers.
This PR ensures that no error is being thrown in this case and also adds core-6.0.2-windows as an alternative reference if the user used only desktop-5.1.14393.206-windows in the rule settings.

PR Checklist

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[JamesWTruher]

my dislike of the test is not a blocker