Geo: Add utiltity to clean up old-style temporary repositories

Note: This MR is not meant to be merged - it is a utility only for a specific run on production. If it needs to be made available to customers at some point, this can be re-opened.

What does this MR do?

Add a rake task to remove any old geo temporary directories

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

A previous Geo version stored temporary git repositories with a special naming convention, such as www-gitlab-com_8af8612d0fb137.git. We now store these in the @geo-temporary directory. But several old style dirs could be hanging around, taking up space.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • [-] Has been reviewed by a UX Designer
  • [-] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [-] Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [-] Internationalization required/considered
• [-] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-com/migration#322 (closed)