infra: use new del_branch_on_merge in .asf.yaml (#2788)

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# Rationale for this change
Previous use of `del_branch_on_merge` 
```
github:
  del_branch_on_merge: true
```
is deprecated,
https://github.com/apache/infrastructure-asfyaml?tab=readme-ov-file#delete-branch-on-merge

New way is to 
```
github:
  pull_requests:
    del_branch_on_merge: true
```

https://github.com/apache/infrastructure-asfyaml?tab=readme-ov-file#pull_requests

## Are these changes tested?

## Are there any user-facing changes?

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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