Build ref-protection against gerrit stable-3.6

Bug: Issue 15831
Change-Id: I9821d8b603366b3d8ab9aac0de4532a24f01e905
diff --git a/jenkins/gerrit-bazel-plugin-ref-protection.yaml b/jenkins/gerrit-bazel-plugin-ref-protection.yaml
index 210e064..0797379 100644
--- a/jenkins/gerrit-bazel-plugin-ref-protection.yaml
+++ b/jenkins/gerrit-bazel-plugin-ref-protection.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3