fix: update Mergify queue configuration

[KirtiRamchandani]

Description

Mergify currently marks pull requests with failed Summary / Mergify Merge Queue checks because the queue action contains fields that are no longer accepted:

• actions.queue.method
• actions.queue.commit_message_template

This moves the squash strategy and equivalent commit-message behavior onto the shared queue_rules entry instead:

• merge_method: squash
• commit_message_format: with title: pr-title and body: pr-body

That preserves the existing {{ title }} (#{{ number }})\n\n{{ body }} behavior using Mergify's current declarative format.

Validation

• ruby -e "require 'yaml'; data=YAML.load_file('.mergify.yml'); puts data['queue_rules'][0]['merge_method']; p data['queue_rules'][0]['commit_message_format']"
• git diff --check

[KirtiRamchandani]

Mergify has validated the replacement config on this PR:

• Configuration changed: success
• output title: The new Mergify configuration is valid
• output summary: This new configuration will be used for the next pull requests once it is merged.

The remaining failed Summary / Mergify Merge Queue checks are still reporting the current default-branch configuration error, which is the issue this PR removes.