Ensure backup restores clears storages before restoring

What does this MR do and why?

gitaly#4748 (closed)

This MR tells gitaly-backup which storages should be cleared out before starting a full restore. This prevents a timing issue as described in #300974 (comment 506439725)

How to set up and validate locally

These instructions should work through praefect or just gitaly - basically it shouldn't matter how your GDK is setup.

1. Ensure this branch is checked-out, and restart GDK.

   $ git switch jf_gitaly_backup_remove_all_repositories
   $ gdk restart
   $ bundle exec rails db:migrate RAILS_ENV=development

2. Create a backup:

   $ bundle exec rake gitlab:backup:create
   ...
   2023-03-29 23:48:42 UTC -- Backup 1680133628_2023_03_29_15.11.0-pre is done.
   2023-03-30 12:48:42 +1300 -- Deleting backup and restore lock file

3. Create a new project with repository.
4. Restore from the backup:

   $ bundle exec rake gitlab:backup:restore BACKUP=1680133628_2023_03_29_15.11.0-pre
   ...

5. Create a new project with repository. If this works without error, then the bug has been fixed.

MR acceptance checklist

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