net-mgmt/zabbix5-server: Update to 5.0.8

Changelog: https://www.zabbix.com/rn/rn5.0.8

And while here, also change PORTVERSION to DISTVERSION where apropriate.

PR: 253017
Submitted by: pg@pakhom.spb.ru (maintainer)
Reviewed by: osa (mentor)
Approved by: osa (mentor)
Differential Revision: https://reviews.freebsd.org/D28352