Increase GIT_DEPTH for the "assign-milestones" job

Cloning tags with the default GIT_DEPTH of 1 prevents the milestone
assignment script from identifying any merge requests that are included
in a given release.  Fix by increasing GIT_DEPTH to an arbitrary value
that is high enough for practical purposes.

(cherry picked from commit bac4a57759)

.gitlab-ci.yml

@@ -1959,6 +1959,8 @@ assign-milestones:
   needs:
     - job: sign
       artifacts: false
+  variables:
+    GIT_DEPTH: 1000
   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