task: simplify the example of TaskTracker

[Tudyx]

This PR uses with_cancellation_token_owened inside the TaskTracker documentation. This avoids using the select! macro. The rationale is one described inside the select! documentation :

The select! macro is a powerful tool for managing multiple asynchronous branches, enabling tasks to run concurrently within the same thread. However, its use can introduce challenges, particularly around cancellation safety, which can lead to subtle and hard-to-debug errors. For many use cases, ecosystem alternatives may be preferable as they mitigate these concerns by offering clearer syntax, more predictable control flow, and reducing the need to manually handle issues like fuse semantics or cancellation safety.

[ADD-SP]

Thanks!