Backup UI lists deleted applications as still included in backup schedule

[nrauso]

Deleted applications can remain listed in the backup UI as if they were still included in an existing backup schedule.
The backup details dialog and the backup card still show those applications, even though they no longer exist in the cluster.
The same stale references are also returned by list-backups.

Steps to reproduce

• Start with a cluster containing two app instances, for example nethvoice1 and nethvoice2;
• Create a backup schedule including only nethvoice1 and run it;
• Edit the backup schedule and add nethvoice2 and run it;
• Remove nethvoice2 from the cluster;
• Open the backup page and check the existing backup schedule.

Expected behavior

Deleted applications should not be shown as applications included in a backup schedule.

Actual behavior

Deleted applications are still shown in the backup UI as backed up applications.

Components

• core:3.20.0

See also

• Discussion https://mattermost.nethesis.it/nethesis/pl/wkxhbugfrpdfbqj7wr6odfadua