MariaDB 10.3 rollout

We just included MariaDB 10.3. If you run ./update.sh, you will encounter some errors in mysql-mailcow, that will be fixed by the upgrade process triggered by php-fpm-mailcow.
A SQL backup is recommended.

EN: If you have a mailcow support subscription, feel free to use the ticket system for assistance or help after a failed update.
DE: Falls ein mailcow Supportpaket gebucht wurde, helfen wir gerne beim Update der mailcow Installation oder bei etwaigen Problemen.