Adding stable-3.9 build

Change-Id: I411a90f03e4c5d565508c10f4d43f2824f73dca8
diff --git a/jenkins/gerrit-mvn-plugin-ai-code-review.yaml b/jenkins/gerrit-mvn-plugin-ai-code-review.yaml
index 3088ca4..25cfe13 100644
--- a/jenkins/gerrit-mvn-plugin-ai-code-review.yaml
+++ b/jenkins/gerrit-mvn-plugin-ai-code-review.yaml
@@ -4,5 +4,6 @@
     branch:
       - master
       - stable-3.8
+      - stable-3.9
     jobs:
       - 'plugin-{repo}-mvn-{branch}'