Support restoring repository backups in parallel

What does this MR do?

#35862 (closed)

Concurrent restores were added to gitaly-backup in gitaly!3796 (merged) without also changing gitlab-rails this concurrency would not be controllable.

How to setup and validate locally (strongly suggested)

1. Make sure GDK is up-to-date. Without the requisite changes in gitaly, trying to restore with concurrency will result in an error.
2. Take a backup (shell):

   $ bundle exec rake gitlab:backup:create
   ...
   Creating backup archive: 1630989194_2021_09_07_14.3.0-pre_gitlab_backup.tar ... done

3. Restore the backup with concurrency (shell):

   $ be rake gitlab:backup:restore GITLAB_BACKUP_MAX_CONCURRENCY=4 GITLAB_BACKUP_MAX_STORAGE_CONCURRENCY=1 BACKUP=1630989194_2021_09_07_14.3.0-pre

   If repository restores begin, the change has succeeded.

Does this MR meet the acceptance criteria?

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Security

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