Enable rebase_on_merge_automatic feature flag by default

Marc Shawrequested to merge
enable-rebase-on-merge-automatic-ff-default into master

What does this MR do and why?

Enables the rebase_on_merge_automatic feature flag by default by changing the flag type from gitlab_com_derisk to beta and setting default_enabled: true.

This feature allows GitLab to automatically rebase the source branch onto the target branch at merge time when using Merge commit with semi-linear history or Fast-forward merge methods. It was introduced in GitLab 18.0 and has been successfully derisk-tested on GitLab.com.

Related issue: #519381 (closed) Rollout issue: #524048 (closed)

MR acceptance checklist

  • Feature flag type changed from gitlab_com_derisk to beta
  • default_enabled set to true
  • Documentation updated with history entry
Edited by Marc Shaw

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