Allow custom backup rotation

[qk-santi]

Is your feature request related to a problem? Please describe.

Default backup rotation is "fine", but many people would have different environments.
Some people may want to backup changes for 2 years, while others don't care for anything older than 30 days.
It can greatly depend on the importance of the data stored.
Also, this can dictate a lot the backup storage space needs, so not being this configurable, may render some options unusuable.
Can't this be made configurable?

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[szaimen]

This will be implemented with #2428

[szaimen]

This is now released with v6.0.0 Beta. Testing and feedback is welcome! See https://github.com/nextcloud/all-in-one#how-to-switch-the-channel