[9.18] new: ci: Add "assign-milestones" job to CI

After a release is signed (the "sign" job), the "assign-milestones" job
will assign issues and merge requests to the corresponding version
milestones.

Backport of MR !11868

Merge branch 'backport-andoni/add-assign-milestone-ci-job-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11872

.gitlab-ci.yml

@@ -1930,6 +1930,19 @@ publish-cleanup:
   tags:
     - smalljob
 
+# Job assigning issues and merge requests to version milestones
+
+assign-milestones:
+  <<: *manual_release_job_qa
+  needs:
+    - job: sign
+      artifacts: false
+  script:
+    - |
+      "${CI_PROJECT_DIR}"/bind9-qa/releng/assign_mrs_to_versioned_milestones.py --version "${CI_COMMIT_TAG}" --metadata "${CI_PROJECT_DIR}"/bind9-qa/releng/metadata.json
+  rules:
+    - *rule_tag
+
 # Jobs to update RPMs
 
 .rpm-build-job: &rpm_build_job