Update release steps for version checks
What does this MR do and why?
Updates release steps for manual version-related checks.
We made major changes to versioning in 15.2 when we made the Versions menu dynamic, then again in 15.6 when we introduced the GitLab Docs Archives site. This MR updates the directions for checking the contents of the versions dropdown menu to reflect these changes.
Merge request acceptance checklist
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I have evaluated the MR acceptance checklist for this merge request.