Support unmaintained branches in prepare-runner workflow

Author: markgoddard

roles/github/templates/prepare-runner.yml.j2

@@ -25,4 +25,4 @@ jobs:
         id: openstack_release
         run: |
           BRANCH=$(awk -F'=' '/defaultbranch/ {print $2}' .gitreview)
-          echo "openstack_release=${BRANCH}" | sed "s|stable/||" >> $GITHUB_OUTPUT
+          echo "openstack_release=${BRANCH}" | sed -E "s,(stable|unmaintained)/,," >> $GITHUB_OUTPUT