Merge branch 'stable-3.1' of git.ispconfig.org:ispconfig/ispconfig3 into stable-3.1

install/dist/conf/centos80.conf.php

install/dist/lib/centos80.lib.php

install/lib/install.lib.php

interface/lib/classes/aps_guicontroller.inc.php

interface/web/dashboard/lib/lang/it.lng

interface/web/dashboard/lib/lang/it_dashlet_customer.lng

...rface/web/dashboard/lib/lang/it_dashlet_databasequota.lng

interface/web/dashboard/lib/lang/it_dashlet_limits.lng

interface/web/dashboard/lib/lang/it_dashlet_mailquota.lng

interface/web/dashboard/lib/lang/it_dashlet_modules.lng

interface/web/dashboard/lib/lang/it_dashlet_quota.lng

interface/web/login/lib/lang/it.lng

server/lib/classes/cron.d/100-monitor_system_update.inc.php

server/lib/classes/monitor_tools.inc.php