[9.20] new: ci: Add a check for defined but never used anchors in .gitlab-ci.yaml

If an new orphan anchor is (`.anchor: &anchor` with no corresponding `*anchor` elsewhere in the file) is introduced the CI job will.

Depends on https://gitlab.isc.org/isc-projects/bind9-qa/-/merge_requests/101 (merge that first and then drop the `--branch` commit).

Backport of MR !10654

Merge branch 'backport-stepan/ci-orphaned-anchors-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10666