PSReservedParams: Make severity Error instead of Warning

PR Summary

PSScriptAnalyzer reports PSReservedParams as warning but according to the documentation it's an error.

This PR makes the rule report as an error and fixes up tests that expect there to be 7 rules with an error severity (this makes it 8).

Resolves #1973

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[x] Summarized changes
[x] Change is not breaking
[x] Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
[x] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
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