[9.20] fix: ci: Run merged-metadata job for release branches in private repo

[9.20] fix: ci: Run merged-metadata job for release branches in private repo

The prior regex didn't match the actual names we use for release
branches in the private repo. This caused the merged-metadata job to not
be created upon merging to a release branch, resulting in the private MR
not being properly milestoned.

Use the correct regex along with protecting the v9.*-release branches in
the gitlab UI so that they have access to the token used to perform the
required API operations.

Backport of MR !10003

Merge branch 'backport-nicki/ci-fix-post-merge-in-private-repo-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10013