net-mgmt/unifi-poller: Mark as BROKEN, deprecate and set expiration date to 2021-12-31

This project is now called unpoller and port also is unfetchable.
References: https://github.com/unpoller/unpoller and pkg-fallout

PR: 258341
Approved by: port maintainer timeout (3 weeks), arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D32216