Document prefer mode in 18.9 release notes and deprecation docs

The interlock issue #589497 requires 18.9 release notes and deprecation docs that reference:

• Prefer mode as the path to online GC and future legacy removal
• 19.0 target for default-on in Omnibus (Charts removed from scope per gitlab-com/Product#14345)
• Dependency on Geo backup/restore support for end-to-end safety

The release post is in review, but the release notes and deprecation page entries have not been created.

Parent: #589497