Enable Rapid Diffs by default on the merge request Changes tab

Summary

Make Rapid Diffs the default experience on the merge request Changes tab. Today the rapid_diffs_on_mr_show feature flag is default_enabled: true, but users still see legacy diffs unless they opt in via the in-MR toggle (which sets the rapid_diffs_enabled cookie). Flip the default so Rapid Diffs renders without opt-in, and the toggle becomes an opt-out back to legacy.

Current behavior

Projects::MergeRequestsController#rapid_diffs_page_enabled? (app/controllers/projects/merge_requests_controller.rb:751):

::Feature.enabled?(:rapid_diffs_on_mr_show, current_user, type: :beta) &&
  params[:rapid_diffs_disabled] != 'true' &&
  (params[:rapid_diffs] == 'true' || cookies[:rapid_diffs_enabled] == 'true')

An absent rapid_diffs_enabled cookie ⇒ legacy diffs.

Proposal

Change the default resolution so that, with the feature flag enabled, Rapid Diffs renders unless the user has explicitly opted out:

• Absent/unset toggle ⇒ Rapid Diffs (new default).
• Toggle off ⇒ legacy diffs (opt-out, e.g. rapid_diffs_enabled == 'false' / rapid_diffs_disabled).
• Keep params[:rapid_diffs] / params[:rapid_diffs_disabled] overrides working.

Update the toggle UI copy/state to reflect that Rapid Diffs is the default and the switch now disables it.

Acceptance criteria

• With rapid_diffs_on_mr_show enabled and no prior preference, the MR Changes tab renders Rapid Diffs.
• Users who previously toggled Rapid Diffs off continue to see legacy diffs (opt-out preserved).
• The in-MR toggle switches back to legacy diffs and persists that choice.
• params[:rapid_diffs] / params[:rapid_diffs_disabled] overrides still work.
• Specs updated for the new default.