[CBBS] Handle unknown tasks properly

Currently, an active repository will completely stop/break if it encounters a task type which it doesn't know about. This is not currently a problem, but it may be once backup retention (MB-44863) is introduced, since that will introduce a new command ("expire"). For example, if a customer sets up Backup Retention, and later, as part of a downgrade, they add an older node which doesn't have the code to handle Backup Retention.

While the Backup Service is not officially supported during mixed version mode, backups should still run, especially since this is a situation where backups are more likely to be important.