Build high-availability against gerrit stable-3.6

Bug: Issue 15831
Change-Id: I291798b403239b815be95297e51949b68cb8c37a
diff --git a/jenkins/gerrit-bazel-plugin-high-availability.yaml b/jenkins/gerrit-bazel-plugin-high-availability.yaml
index 4806c22..117e38b 100644
--- a/jenkins/gerrit-bazel-plugin-high-availability.yaml
+++ b/jenkins/gerrit-bazel-plugin-high-availability.yaml
@@ -10,5 +10,6 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.6
             - stable-3.5
       - plugin-{name}-verifier