[Nextcloud] maintenance window configuration

[EV21]

The recently released Nextcloud 28 asks to configure a maintenance start time. This setting will delay daily cron.php taks to the configured time.

Have a look at the official documentation: https://docs.nextcloud.com/server/28/admin_manual/configuration_server/background_jobs_configuration.html

Maybe the Uberspace team would like to discuss how to set this in the guide to avoid spikes in their monitoring at 01:00 UTC.