Address cancelled checks separately with check-cancelled

Mergify now provides a check-cancelled condition attribute that matches checks reporting a cancelled conclusion — for example GitHub Actions jobs stopped by concurrency: cancel-in-progress. Cancelled checks are still matched by check-failure for backward compatibility, but you can now exclude them from failure-based rules by combining check-failure=<check> with -check-cancelled=<check>, so a cancellation no longer triggers actions meant for genuine failures.