Enable wiki immersive editor by default

What does this MR do and why?

This MR updates the wiki_immersive_editor feature flag to move from wip to beta and be default enabled.

References

gitlab-org#20492

Screenshots or screen recordings

Before After
Screenshot_2026-04-16_at_11.05.20 Screenshot_2026-04-16_at_11.04.31

How to set up and validate locally

  1. Open or create a Wiki
  2. Edit a wiki page
  3. Observe the new editing interface

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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