Add a tag pipeline CI job to update the stable tag after a release

Introduce a CI job to update the "stable" tag with the latest released
stable version tag. This is a manual job that only runs in tag
pipelines in the context of a release.

(cherry picked from commit 2bca6c0d5e)

.gitlab-ci.yml

@@ -1986,6 +1986,19 @@ merge-tag:
       - bind9/
     when: on_failure
 
+update-stable-tag:
+  <<: *manual_release_job_qa
+  variables:
+    GIT_DEPTH: 1
+  script:
+    - bind9-qa/releng/update_stable_tag.py --tag "$CI_COMMIT_TAG"
+  rules:
+    - *rule_tag_open_source
+  artifacts:
+    paths:
+      - bind9/
+    when: on_failure
+
 .customer-git: &customer_git
   <<: *base_image
   stage: release