Enable Rapid Diffs by default on the merge request Changes tab

Rapid Diffs becomes the default on the merge request Changes tab everywhere, not only on GitLab.com. This flips rapid_diffs_default_on_mr_show to default_enabled: true.

gitlab_com_derisk flags can not be default enabled, so the flag graduates to beta and its definition moves to config/feature_flags/beta/. I kept the flag instead of removing it so Self-Managed instances have a way to turn this back off.

The rapid_diffs_on_mr_show beta flag still gates availability, so nothing changes on instances where Rapid Diffs is off. Users can still opt out with the diff view toggle or with ?rapid_diffs_disabled=true.

The global stub_feature_flags(rapid_diffs_default_on_mr_show: false) in spec/spec_helper.rb stays for now, since the merge request spec suite migration is still in flight in !247832 (merged). Only the stale comment above it is reworded.

Reproduction steps

1. Open the Changes tab of any merge request with no rapid_diffs_enabled cookie set. Rapid Diffs renders.
2. Switch back to legacy diffs with the diff view toggle, then reload the page. Legacy diffs persist.
3. Open the same tab with ?rapid_diffs_disabled=true. Legacy diffs render.

Related

• Feature issue: #602723 (closed)
• Rollout issue, closed at 100% of actors: #609355 (closed)
• Introduced by: !247629 (merged)
• Spec suite migration: !247832 (merged)