Deprecate rake gitlab:list_repos

What does this MR do and why?

An old, incomplete, not-recommended procedure for migrating GitLab instances instructs you to use rake gitlab:list_repos. That rake task has only raised an error for the last 2 years. The existence of this rake task blocks some Gitaly effort until it is removed.

This MR deprecates rake gitlab:list_repos.

Part of #384361 (closed)

Related to #350080 (closed)

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