Stop building automerger for stable-3.8/9 using master branch

The master branch doesn't support gerrit 3.8/3.9.
Instead build the actual branches that exist.

Change-Id: Ie63bad1d8cc192f6ccaa2b138012b7475bfd7327
diff --git a/jenkins/gerrit-bazel-plugin-automerger.yaml b/jenkins/gerrit-bazel-plugin-automerger.yaml
index 24263ae..d864295 100644
--- a/jenkins/gerrit-bazel-plugin-automerger.yaml
+++ b/jenkins/gerrit-bazel-plugin-automerger.yaml
@@ -6,8 +6,8 @@
             - master
           gerrit-branch:
             - stable-3.10
-            - stable-3.9
-            - stable-3.8
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.9
+            - stable-3.8