Merge branch '6328-support-php-8-x' into 'develop'

install/install.php

install/lib/classes/tpl.inc.php

install/lib/installer_base.lib.php

install/lib/mysql.lib.php

install/tpl/config.inc.php.master

install/update.php

interface/lib/app.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/lib/classes/validate_datetime.inc.php

interface/lib/config.inc.php

interface/web/client/client_edit.php

interface/web/dashboard/dashlets/limits.php

interface/web/mailuser/mail_user_autoresponder_edit.php

server/lib/app.inc.php