net-mgmt/zabbix6-server: Make service depend on mysql service

When using zabbix6-server with MariaDB, rc.d/zabbix_server does not
contain "REQUIRE: mysql". Therefore, when shutting down the host, mysql
stops before zabbix_server and prevents zabbix_server from stopping.

Fix this also for MYSQLDV option and also for zabbix6-proxy in
addition to zabbix6-server.

PR: 278629