Document `gitlab:backup:` rake tasks for Omnibus users <12.2

GitLab Backup and Restore documentation was updated to replace gitlab:backup:create and gitlab:backup:restore with gitlab-backup command equivalent for all Omnibus users.

Problems:

• Omnibus <12.2 users will see gitlab-backup: command not found when issuing gitlab-backup commands
• without documentation of the gitlab:backup:create and gitlab:backup:restore rake tasks in Omnibus <12.2, users will have difficulty discovering/using the Omnibus-included gitlab:backup:create/restore rake tasks.
• this blocks Support's ability to point customers using Omnibus <12.2 to working backup/restore instructions.

This is particularly concerning as the inclusion of gitlab-backup is slated for 12.2 milestone which will not be released until Aug 22nd. Until then, our backup/restore documentation will be inaccurate and error-prone for all Omnibus users.

https://docs.gitlab.com/ce/raketasks/backup_restore.html

One potential solution would be to suggest two command options for Omnibus users in each situation - one for 12.2+ and one for "legacy" users. Example

If you're using Omnibus 12.2 or newer, use
`gitlab-backup create`

If you're using Omnibus 12.1 or older, use
`gitlab-rake gitlab:backup:create`

This change was introduced here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31555/diffs as part of omnibus-gitlab!3447 (merged)