feat: sunsetting legacy editor

[doc-han]

Description

This PR sunsets the legacy editor

Closes #4402

Validation steps

1. (How can a reviewer validate your work?)

Additional notes for the reviewer

1. (Is there anything else the reviewer should know or look out for?)

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

You can read more details in our
Responsible AI Policy

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR