Drop FreeBSD and OpenBSD from CI

Both FreeBSD and OpenBSD in the CI are tested on outdated images.
Current FreeBSD images can't even be rebuild because in the maintained
branches they were ported from QCOW2 to the AWS autoscaler (also a
future of the OpenBSD image). This is something we don't want to
backport to EoL branches.

Merge branch 'mnowak/drop-bsd-images-from-ci' into 'bind-9.16'

See merge request isc-projects/bind9!10229