Build uploadvalidator plugin on its own stable-3.1 branch

Change-Id: I03b95c26da2236406155367ecf6a15821fae483b
diff --git a/jenkins/gerrit-bazel-plugin-uploadvalidator.yaml b/jenkins/gerrit-bazel-plugin-uploadvalidator.yaml
index 1006c5c..f225b70 100644
--- a/jenkins/gerrit-bazel-plugin-uploadvalidator.yaml
+++ b/jenkins/gerrit-bazel-plugin-uploadvalidator.yaml
@@ -3,6 +3,7 @@
     jobs:
       - 'plugin-{name}-bazel-{branch}':
           branch:
+            - stable-3.1
             - stable-3.0
             - stable-2.16
       - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
@@ -11,4 +12,3 @@
           gerrit-branch:
             - master
             - stable-3.2
-            - stable-3.1