Build high-availabililty plulgin on stable-3.9

Change-Id: I8b40073fcef595d5ce48b5719fee4ed8e0b4a88a
diff --git a/jenkins/gerrit-bazel-plugin-high-availability.yaml b/jenkins/gerrit-bazel-plugin-high-availability.yaml
index 0f3c439..8ab48f1 100644
--- a/jenkins/gerrit-bazel-plugin-high-availability.yaml
+++ b/jenkins/gerrit-bazel-plugin-high-availability.yaml
@@ -5,6 +5,7 @@
           extra-modules: 'global-refdb'
           branch:
             - master
+            - stable-3.9
             - stable-3.8
             - stable-3.7
       - plugin-{name}-verifier