net-mgmt/zabbix6-server: Update to 6.0.6

Though this commit "only" updates net-mgmt/zabbix6-server to 6.0.6, it
effectively updates all net-mgmt/zabbix6-* to 6.0.6.

PHP 8.0 and 8.1 are supported though it is not sure how fully they are.
The release notes discuss both.

zabbix6 also supports MariaDB 10.7. Note that MariaDB 10.7 support will
be added to the zabbix6 family of ports when MariaDB is added to ports.

Release notes can be found at https://www.zabbix.com/rn/rn6.0.6.