net/zebra: Update MASTER_SITES, deprecate and set expiration date to 2021-12-31

Remove dead mirrors, Zebra's last release was back in 2005 and is
abandoned upstream

Reference: https://www.gnu.org/software/zebra/

Approved by:	bdrewery (port maintainer), arrowd (mentor)
Differential Revision:	https://reviews.freebsd.org/D32469