[DO NOT MERGE] Pester 6.0.0 release candidate update

[nohwnd]

Hi — Pester 6.0.0 is almost ready, and before I release it I'm trying it out
against real-world Pester v5 test suites.

This PR updates Pester to the 6.0.0 release candidate and makes the changes
needed for your tests to run on it. I opened it to learn two things:

• whether migrating from v5 to v6 is straightforward, and
• where v6 breaks or behaves differently, so I can fix those things before the
  final release instead of after.

You don't need to merge this, and I'd suggest you don't — it's only meant to
show what a v6 update would involve and to give me feedback. Please leave it
open, though: if more release candidates come out I'll push those updates here
too, and I'll close it myself afterwards.

This PR was co-authored by GitHub Copilot. If you'd rather not receive
AI-assisted PRs, just close it and I won't open one for the next release.

Thanks for the CI time this run used, and for maintaining a suite I could test
against.

— Jakub & Copilot

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