Add new unified GitLab 16 changes document to global navigation

What does this MR do and why?

For technical-writing#843 (closed), add to the global navigation a new entry for a new file added in: gitlab!128331 (merged).

Also remove a now unneeded entry for the old omnibus-gitlab page.

Screenshots, screen recordings, or links to review app

Review app: https://eread-add-new-unified-gitlab-16-changes-document-to-global-nav.docs.gitlab-review.app/ee/update/versions/gitlab_16_changes.html

Merge request 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 merge request.