Implemented #5651 Make ISPConfig code base compatible with PHP 7.4

install/lib/install.lib.php

install/lib/installer_base.lib.php

install/lib/mysql.lib.php

interface/lib/classes/cmstree.inc.php

interface/lib/classes/db_mysql.inc.php

interface/lib/classes/simplepie.inc.php

interface/lib/classes/system.inc.php

interface/web/mailuser/mail_user_autoresponder_edit.php

interface/web/remote/monitor.php

server/lib/classes/db_mysql.inc.php

server/lib/classes/file.inc.php

server/lib/classes/system.inc.php