infra: automatically delete branch on pr merge (apache#21033)

Enable automatic deletion of head branches after merging.

## Which issue does this PR close?

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- Closes #.

## Rationale for this change


As described in
https://github.com/apache/infrastructure-asfyaml?tab=readme-ov-file#pull_requests
```
github:
  pull_requests:
    del_branch_on_merge: true
```

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Author: kevinjqliu

.asf.yaml

@@ -69,6 +69,8 @@ github:
     # enable updating head branches of pull requests
     allow_update_branch: true
     allow_auto_merge: true
+    # auto-delete head branches after being merged
+    del_branch_on_merge: true
 
 # publishes the content of the `asf-site` branch to
 # https://datafusion.apache.org/