[CBM] Backing up an upgraded cluster that has a previous backup fails due to the _system scope

What is the problem?
Hyun-Ju Vega found that if she:

• backed up a cluster on 7.2
• upgraded the cluster to 7.6
• tried to back up again

she got the following error:

Error backing up cluster: 'travel-sample._system' does not exist on the target cluster; you must exclude it (--include-data/--exclude-data)

This error originates from here and it is to catch the case where you try to restore a system collection (e.g. _default) to a cluster where it does not exist.

What is the solution?
We should disable this check in a backup - you should always be able to backup any collection, even if it didn't exist in a previous backup.