Pin major version of Pester to 4 and upgrade Pester version for WMF4 build

[bergmeister]

PR Summary

PSSA is not ready for Pester v5, therefore pinning it to v4 for the moment to avoid the inevitable build failure next week

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
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