PSReservedParams: Make severity Error instead of Warning

[liamjpeters]

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

PR Checklist

• [x] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [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
• [x] This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.