commit_message_template is deprecated — use commit_message_format

The Jinja2-based commit_message_template setting (on both the merge action and queue_rules) is now deprecated in favor of the declarative commit_message_format. Mergify shows a deprecation notice whenever the setting is used, and it will be removed on September 30, 2026.

For the common merge-commit shapes, the equivalent commit_message_format settings are documented in the migration guide.