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.

Merge branch 'nicki/ci-fix-post-merge-in-private-repo' into 'main'

See merge request isc-projects/bind9!10003

.gitlab-ci.yml

@@ -1827,6 +1827,6 @@ backports:
 merged-metadata:
   <<: *post_merge
   rules:
-    - if: '$CI_PIPELINE_SOURCE == "push" && ($CI_COMMIT_REF_NAME =~ /^bind-9.[0-9]+(-sub)?$/ || $CI_COMMIT_REF_NAME =~ /^bind-9.[0-9]+.[0-9]+-release$/ || $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH)'
+    - if: '$CI_PIPELINE_SOURCE == "push" && ($CI_COMMIT_REF_NAME =~ /^bind-9.[0-9]+(-sub)?$/ || $CI_COMMIT_REF_NAME =~ /^v9.[0-9]+.[0-9]+-release$/ || $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH)'
   script:
     - bind9-qa/releng/after_merge.py "$CI_PROJECT_ID" "$MERGE_REQUEST_ID"