[9.18] rem: ci: Drop OpenBSD from the CI

With the ongoing process of moving CI workloads to AWS, OpenBSD poses a
challenge, as there is no OpenBSD AMI image in the AWS catalog. Building
our image from scratch is disproportionately complicated, given that
OpenBSD is not a common deployment platform for BIND 9. Otherwise,
OpenBSD stays at the "Best-Effort" level of support.

Backport of MR !10375

Merge branch 'backport-mnowak/drop-openbsd-from-ci-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10429