[9.18] new: ci: Merge tag back to its base branch in tag pipeline

Add a CI job to merge tags back to the respective base branch in tag pipelines.

Backport of MR !11001

Merge branch 'backport-andoni/add-merge-tag-to-tag-pipeline-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11025

.gitlab-ci.yml

@@ -1632,6 +1632,26 @@ sign:
   when: manual
   allow_failure: false
 
+# Job merging the tag back into its base branch
+
+merge-tag:
+  <<: *base_image
+  stage: release
+  when: manual
+  variables:
+    GIT_DEPTH: 100
+  before_script:
+    - git clone --depth 1 https://gitlab.isc.org/isc-projects/bind9-qa.git
+  script:
+    - bind9-qa/releng/merge_tag.py --tag "$CI_COMMIT_TAG"
+  needs: []
+  rules:
+    - if: '$CI_COMMIT_TAG != null && $CI_COMMIT_TAG !~ /-S/'
+  artifacts:
+    paths:
+      - bind9/
+    when: on_failure
+
 .customer-git: &customer_git
   <<: *base_image
   stage: release