Consistently load backup manifest

What does this MR do and why?

#351383 (closed)

Previously it was not clear whether one should use settings or backup_information. This resulted in both being used inconsistently. Instead of silently loading backup information, we explicitly load or build the information depending.

Now that we can rely on backup information being consistent, this means we can now start using it to detect when tasks are skipped instead of adhoc reading from ENV.

Eventually we'll use this consistency to be able to pass a backup ID to the backup tasks.

How to set up and validate locally

1. Create a backup.

   $ bundle exec rake gitlab:backup:create
   ...
   Creating backup archive: 1644531746_2022_02_11_14.8.0-pre_gitlab_backup.tar ... done
   Uploading backup archive to remote storage  ... skipped
   Deleting tmp directories ... done
   done
   done
   done
   done
   done
   done
   done
   done
   done
   Deleting old backups ... skipping
   Warning: Your gitlab.rb and gitlab-secrets.json files contain sensitive data 
   and are not included in this backup. You will need these files to restore a backup.
   Please back them up manually.
   Backup task is done.

2. Restore from the backup. Note that BACKUP does not take a full path.

   $ bundle exec rake gitlab:backup:restore BACKUP=1644531746_2022_02_11_14.8.0-pre
   ...

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