net-mgmt/zabbix6-server: Require mysql 8.0

When using mysql, zabbix6 requires 8.0, not the default 5.7. 8.0 is
supported by upstream.

PR: 262438
Reported by: rob2g2 <rob2g2-freebsd@bitbert.com>

(cherry picked from commit 9d8cb8eb28c5e8f7be50841c27d54cb23db20db1)