Update archives list

What does this MR do and why?

The Archives listing page provides registry URLs for downloading containerized versions of the Docs site.

The endpoint changed in 15.6 when we introduced the Archives site and Lunr for offline search.

• 15.6 and above should use: registry.gitlab.com/gitlab-org/gitlab-docs/archives:$version
• 15.5 and lower should use: registry.gitlab.com/gitlab-org/gitlab-docs:$version

Our current page incorrectly uses the newer endpoint for 15.5. This MR fixes it.

Related: gitlab!144383 (merged)

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