Give explicit error on unsupported versions of PSCore and document it in ReadMe as well

[bergmeister]

PR Summary

Closes #1042 by giving an explicit error on unsupported versions of pscore and document it. It is already documented in the release notes (and the release blog post)
The reason for that is the reference to SMA (which cannot be lower because the 6.0.0 and 6.0.1 NuGet packages had bugs in them that made them not usable for PSSA.

PR Checklist

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