INFRA: Use PR titles for squash commits (#3755)

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# Rationale for this change
Context: apache/iceberg#17467

Configure squash merges to always use the pull request title while
preserving the existing commit-message details in the squash commit
body.

## Are these changes tested?

## Are there any user-facing changes?

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Author: kevinjqliu

.asf.yaml

@@ -31,6 +31,7 @@ github:
   enabled_merge_buttons:
     merge: false
     squash: true
+    squash_commit_message: PR_TITLE_AND_COMMIT_DETAILS
     rebase: true
   protected_branches:
     main: