sysutils/nextcloud-backup: add option for "Backup Count" and prune old backup files

[Starkstromkonsument]

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Is your feature request related to a problem? Please describe. There is no way to automatically prune old backup files.

Describe the solution you'd like Add an option for "Backup Count" alike the on of the Google Drive Backup and prune old backup files accordingly.

Describe alternatives you've considered Delete old Backup files "by Hand".