Draft: Handle bucket prefixes for `rake gitlab:cleanup:remote_upload_files`

What does this MR do and why?

We officially introduced support for prefixes in GitLab 15.0 in !91307 (merged), though users may have been quietly using this for a while.

This Rake task was not updated to take this into account.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/415537

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