Make automatic rebase before merge generally available

What does this MR do and why?

The automatic rebase before merge feature has completed its Beta period and is now generally available. This MR:

• Removes the Beta badge from the Enable automatic rebase prior to merge checkbox in the project merge request settings.
• Corrects the documentation history. The rebase_on_merge_automatic feature flag was removed in GitLab 19.0, but the feature remained Beta after the badge was restored in merge request 234180. The Generally available entry is corrected from 19.0 to 19.2, and the flag removal is kept as a separate 19.0 history entry.

References

• Generally available work item: gitlab-org#16803
• Beta badge previously restored in: !234180 (merged)
• Feature flag and Beta badge originally removed in: !231406 (merged)

Screenshots or screen recordings

Before	After
Checkbox label shows a Beta badge	Checkbox label without the Beta badge

Before/after screenshots to be attached — this is a badge-only visual change to the merge request settings checkbox.

How to set up and validate locally

1. In a project's Settings > Merge requests, select the Merge commit with semi-linear history or Fast-forward merge method.
2. Confirm the Enable automatic rebase prior to merge checkbox no longer displays a Beta badge.