Configure 3.8 and 3.9 builds for the code-owners plugin

The master version of the code owners plugin is no longer
compatible with Gerrit 3.8 and 3.9. Hence the branches
stable-3.8 and stable-3.9 were created for this plugin.
Adapt the build jobs to this change.

Change-Id: Id5b0d2908b7c97b5440e78a79b338d1fda9dbc95
diff --git a/jenkins/gerrit-bazel-plugin-code-owners.yaml b/jenkins/gerrit-bazel-plugin-code-owners.yaml
index 927dadc..f43ca47 100644
--- a/jenkins/gerrit-bazel-plugin-code-owners.yaml
+++ b/jenkins/gerrit-bazel-plugin-code-owners.yaml
@@ -1,12 +1,9 @@
 - project:
     name: code-owners
     jobs:
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
-          branch:
-            - master
-          gerrit-branch:
-            - stable-3.8
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
             - stable-3.7
+            - stable-3.8
+            - stalbe-3.9