Use non-deprecated del_branch_on_merge

Author: kevinjqliu

.asf.yaml

@@ -42,7 +42,9 @@ github:
         required_approving_review_count: 1
 
       required_linear_history: true
-  del_branch_on_merge: true
+  pull_requests:
+    # auto-delete head branches after being merged
+    del_branch_on_merge: true
   features:
     wiki: true
     issues: true