net-mgmt/zabbix6*: Update 6.0.3 --> 6.0.4

See release notes at https://www.zabbix.com/rn/rn6.0.4.

PHP 8.0 is still not consistently supported, however looking at
https://support.zabbix.com/browse/ZBX-20821 users may be able to
test without the IGNORE_WITH_PHP line. The PHP 7.4 requirement will
remain until zabbix 6 release notes are updated.