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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-->PyIceberg is a Python library for programmatic access to Iceberg table metadata as well as to table data in Iceberg format. It is a Python implementation of the Iceberg table spec.
The documentation is available at https://py.iceberg.apache.org/.