net-mgmt/zabbix22-*: Update to 2.2.20

Update CONFLICTS to also match zabbix32. [1]
Improve LICENSE.

Changes: https://www.zabbix.com/rn2.2.20

PR: 222810
PR: 222317 [1]
Approved by: Pakhom Golynga <pg@pakhom.spb.ru> (maintainer)
Submitted by: Franco Fichtner <franco@opnsense.org> [1]