[Website][Dev] Update merge_pr script on arrow-site repository to use new github workflow

[asfimport]

The merge_pr script on the arrow-site repository is still using the old merge workflow. Closing PRs and not using the GITHUB_TOKEN.

We probably should update the script to use the same workflow we use on the main arrow repository.

This was raised when merging this PR apache/arrow-site#225

 

Reporter: Raúl Cumplido / @raulcd

_{Note: This issue was originally created as ARROW-17271. Please see the migration documentation for further details.}

[github-actions]

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