Add allow_update_branch to .asf.yaml

[Dev-iL]

Enable the allow_update_branch setting for the project.

Either true to always allow a pull request head branch that is behind its base branch to be updated even if it is not required to be up to date before merging, or false otherwise.
https://docs.github.com/en/rest/repos/repos?apiVersion=2022-11-28

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[potiuk]

Thanks @Dev-iL for finding it out :D

[github-actions]

Backport successfully created: v3-1-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-1-test