Add documentation for Test-WdacEnablement null reference issue

[erskinejohn]

Document known issue with Test-WdacEnablement failing due to null reference during upgrade validation, including symptoms, resolution steps, and applicable versions.

[Copilot]

Pull request overview

This PR adds a new Environment Validator known-issue troubleshooting guide documenting a Test-WdacEnablement null-reference crash that can block upgrade readiness validation, with steps to identify the condition and apply a temporary workaround.

Changes:

• Adds a new known-issue markdown document describing symptoms and affected scenarios for the Test-WdacEnablement null-reference failure.
• Provides a workaround procedure (identify GUID-named active .cip files and temporarily rename them) and guidance to re-run validation.
• Documents affected/fixed version ranges and includes related Microsoft Learn references.

[aldawson33]

@erskinejohn you have some reviews to answer before this can be approved/merged. Can you please look them over?

[erskinejohn]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved. The TSG/EnvironmentValidator/README.md conflict was resolved by keeping both the WdacEnablement entry (from this PR) and the Test-Cluster Access Denied WMI Error entry (from main). All changes from main have been merged in.