Add branch protection for v3-1-stable (apache#56122)

Now that 3.1.0 is released. `v3-1-stable` should be protected while cherry-picking can happen on `v3-1-test` branch.

Author: kaxil

.asf.yaml

@@ -144,6 +144,11 @@ github:
         required_approving_review_count: 1
       required_linear_history: true
       required_signatures: false
+    v3-1-stable:
+      required_pull_request_reviews:
+        required_approving_review_count: 1
+      required_linear_history: true
+      required_signatures: false
     providers-fab/v1-5:
       required_pull_request_reviews:
         required_approving_review_count: 1