Restore Beta badge on automatic rebase setting

Marc Shawrequested to merge
ms/restore-auto-rebase-beta-badge into master

What

Re-adds the Beta badge next to the Enable automatic rebase prior to merge project setting in the Merge options panel.

Why

The auto-rebase feature benefits from a clearer "still maturing" signal in the UI while we continue to validate behaviour across merge-method combinations.

Screenshots

Before After
No badge Beta badge next to the checkbox

(Visit Settings → Merge requests → Merge options for a project where automatic_rebase_enabled is available to verify locally.)

Checklist

  • Frontend-only change (HAML), no behaviour change
  • No new strings introduced (label string unchanged; badge is an existing component)
  • No spec changes required — existing checkbox-presence specs pass unchanged

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