ci(mergify): upgrade configuration to current format

[mergify]

Hey there! 👋

We've noticed that your Mergify configuration is using some deprecated fields.

No worries—we've got your back! This automated PR updates your config to align with the newest standards, ensuring everything keeps running smoothly.

Do not procrastinate! You must upgrade your configuration before 2026-07-31, otherwise your configuration will stop working.

What's Changed?

• pull_request_rules → actions → delete_head_branch: The delete_head_branch action is deprecated. Use GitHub's native 'Automatically delete head branches' setting instead: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-the-automatic-deletion-of-branches (deadline: 2026-07-31)

Why This Matters

Keeping your configuration up-to-date means you'll benefit from the latest features and improvements Mergify has to offer. Plus, it helps prevent any unexpected hiccups down the road.

Got Questions? We've Got Answers! 🙌

Is this update safe to merge?

Absolutely! We've made sure the changes are compatible with your current setup. Your workflows should continue to work just as before—if not better!

Do I need to do anything special after merging?

Nope! Just merge this PR, and you're all set. If you have any custom configurations, it's a good idea to give them a quick look to ensure everything's in order.

What if I run into issues or have concerns?

We're here to help! Feel free to reach out to our support team anytime.

Thanks for being awesome and keeping your configuration up-to-date! If you have any thoughts or need a hand, don't hesitate to let us know.

Happy merging! 🎉

[mergify]

This pull request does not have a backport label. Could you fix it @mergify[bot]? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.