Clarify global.gitlabVersion usage

What does this MR do?

Attempting to clarify the use of global.gitlabVersion. It is not as straightforward as the docs currently suggest.

Caveats I ran into:

• tags from topic branches are not published to the /cng/* repos but /cng-mirror/*, so they will not work when used as gitlabVersion without also setting image.repository.
• it wasn't immediately clear to me what version actually means or how to know which version tags there are

Related issues

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