Add feature flag for rebase without CI

What does this MR do and why?

Puts the MR !76056 (merged) (community contributor) behind a feature flag due to the feature not working for detached pipelines. See more here #349362 (comment 809294371)

Related to #118825 (closed)

Screenshots or screen recordings

With flag enabled

With flag disabled

How to set up and validate locally

Check with flag and without flag

1. Enable rebase_without_ci_ui
2. Go to a project
3. Set up CI
4. Go to settings
5. Under Merge requests, enable Fast-forward merges
6. Create a merge request towards the default branch
7. Commit a change to the default branch
8. See the new button (see Screenshots in this MR)
9. Click on the chevron-down button directly next to the Rebase button
10. Select Rebase without CI
11. Perform a rebase without triggering CI (beware of #349362 (closed))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.