missing php-sodium optdepends

NC 28 seems to have added it and admin page now complains when it is missing.

https://docs.nextcloud.com/server/28/admin_manual/installation/php_configuration.html#php-modules

Installing the package, editing php.ini and restarting fpm/webserver to enable the extension makes it go away.

At the time of writing, the Nextcloud Arch wiki page is missing this information.