Merge branch...

Merge branch '6821-fix-issue-with-operator-in-php-8-3-in-100-mailbox_stats_hourly-inc-php' into 'develop'

Resolve "Fix issue with ?? operator in PHP 8.3 in 100-mailbox_stats_hourly.inc.php"

Closes #6821

See merge request ispconfig/ispconfig3!1974