Support PHP 8.X

We need to support PHP 8.X for the next Ubuntu release (22.04) - let's start gathering issues with the current code which we can look into. Please comment on this issue if you find a incompatibility.

ToDo:

• Function strftime() is deprecated in /usr/local/ispconfig/server/lib/classes/cron.inc.php (https://www.howtoforge.com/community/threads/disable-deprecated-for-cron-on-debian10.88776/)
• Uncaught Error: Call to undefined function mysqli_init() in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php:83 > apt install php8.1-mysql (need to add check for this to update script)
• stderr: thrown in /usr/local/ispconfig/interface/lib/classes/tpl.inc.php(1366) : eval()'d code on line 173