gitlab: add psql dump to list of backed up items in rake task (team#40518)

howto/gitlab.md

@@ -1709,9 +1709,11 @@ writing*!) **skips** *repositories** and **artifacts**. It contains:
  * GitLab pages (`pages.tar.gz`)
  * some terraform thing (`terraform_state.tar.gz`)
  * uploaded files (`uploads.tar.gz`)
+ * a PostgreSQL database dump (`db/database.sql.gz`)
 
 The backup job is ran nightly.  GitLab also creates a backup on
-upgrade. Those are purged after two weeks by the wrapper script.
+upgrade. Jobs are purged daily, and are assumed to be covered by
+regular Bacula backups.
 
 The backup job does **NOT** contain those components because they take
 up a tremendous amount of disk space, and are already backed up by