PHP 8.1 compat changes in interface part

install/lib/classes/tpl.inc.php

interface/lib/classes/db_mysql.inc.php

interface/lib/classes/listform.inc.php

interface/lib/classes/listform_actions.inc.php

interface/lib/classes/simplepie.inc.php

interface/lib/classes/system.inc.php

interface/lib/classes/tform_base.inc.php

interface/lib/classes/tpl.inc.php

interface/web/dashboard/dashlets/limits.php

server/lib/classes/cron.inc.php

server/lib/classes/tpl.inc.php