Log pages cache clear keys

What does this MR do and why?

To help debug incidents/bugs where we might be having race conditions with the gitlab pages cache.

Related to: gitlab-pages#827 (comment 1269236701)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.