Add Debian 11 (Bullseye) to OS detection code in ISPConfig and update config for Debian testing (Sid)

Update 500-backup_mail.inc.php Disable quota and ACLS when doing a backup of mdbox mailboxes with dsnyc

        - Query to get PHP-FPM socket directory now gets the correct
          directory

We sometimes had one and mostly had two capture groups. Now we always have two capture groups and thus can always use $2 for the redirect target.

Fixes #6144