Adjust `owners` plugin CI configuration

Adapt the `owners` plugin CI configuration so that it uses a dedicated
branch for each Gerrit branch.

Changes:
* remove job that uses `stable-3.6` to build against Gerrit's
  `stable-3.[6|7|8]`
* add `stable-3.[7|8|9]` refs to branch build job (corresponding refs
  were created with the compatible source)

Change-Id: I6ab051b57b59240ab334c2000e9ec7c3d6f2c9d9
diff --git a/jenkins/gerrit-bazel-plugin-owners.yaml b/jenkins/gerrit-bazel-plugin-owners.yaml
index 2887071..2186170 100644
--- a/jenkins/gerrit-bazel-plugin-owners.yaml
+++ b/jenkins/gerrit-bazel-plugin-owners.yaml
@@ -65,15 +65,11 @@
 - project:
     name: owners
     jobs:
-      - 'plugin-owners-bazel-{branch}-{gerrit-branch}':
-          targets: 'plugins/owners plugins/owners-a*'
-          branch:
-            - stable-3.6
-          gerrit-branch:
-            - stable-3.8
-            - stable-3.7
       - 'plugin-owners-bazel-{branch}':
           targets: 'plugins/owners plugins/owners-a*'
           branch:
             - master
+            - stable-3.9
+            - stable-3.8
+            - stable-3.7
             - stable-3.6