Moved to OPT_OUT in 7d14b725, Now, by
removing the feature gates, this is an mandatory feature.

Related issues:
- https://gitlab.com/gitlab-org/gitaly/issues/526
- https://gitlab.com/gitlab-org/gitaly/issues/1194

Closes https://gitlab.com/gitlab-org/gitaly/issues/749
Closes https://gitlab.com/gitlab-org/gitaly/issues/1212
Closes https://gitlab.com/gitlab-org/gitaly/issues/1195

When `Errno::EBUSY` is raised during restore, this could indicate that
the directory being restored into is a mountpoint. In this case we
explain the user how to retry the restore.

Instead of renaming existing storage directories to a timestamped
format, create a timestamped directory in a temporary location and move
the individual files and folders inside storage directories to it. This
helps us with nested storage directories.

To restore all refs from a bundle, we need to use the --mirror option to
ensure that all refs are properly restored. Without this, refs such as
`refs/merge-requests/X` and `refs/keep-around/Y` would be omitted, which
exposed a bug as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/38319#note_41648608.

Closes #38352