value to enable/disable upgrade-backup in config.php for updater.phar and web-updater

[devnull4242]

Even if it is not recommended, the webupdater is used and offered by Nextcloud. Under certain circumstances, the creation of a backup during the Nextcloud upgrade is not desired. The option "--no-backup" is offered for "updater.phar".
Example post: https://help.nextcloud.com/t/unable-to-complete-nextcloud-update-failed-on-back-up/183982/3

It would be nice if there would be the possibility to specify this option in config.php in general, so that it applies directly to "updater.phar" but above all to the web updater.