Documentation: refs cleanup is enabled since 16.6

What does this MR do and why?

Merge request refs cleanup is enabled since 16.6

It was introduced in 13.4, but behind a feature flag since 16.6.

Closes #429894 (closed)

Relates to #336070 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before:

| Introduced in GitLab 13.4, 14 days after a merge request gets closed or merged, the merge request head ref is deleted. This means that the merge request isn't available for local checkout from the merge request head ref anymore.

After:

| Since GitLab 16.6, 14 days after a merge request gets closed or merged ...

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.