Add note about modification of gitlab-secrets in 15.4

What does this MR do and why?

We have modified the structure of gitlab-secrets.json in GitLab 15.4, see the MR omnibus-gitlab!6310 (merged) for details.

While the modification is not critical, it makes sense to re-sync the files across nodes when using GitLab HA or Geo. This MR adds a note about it to the page Version-specific upgrade instructions.

Detailed discussion can be found in this slack thread (subject to 90 retention).

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