### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
self,
        *,
        task_id: str,
        map_indexes: Collection[int] | None = None,
        execution_date: datetime | None = None,
        run_id: str | None = None,
        state: TaskInstanceState,
        upstream: bool = False,
        downstream: bool = False,
        future: bool = False,
        past: bool = False,
        commit: bool = True,
        session=NEW_SESSION,
    ) -> list[TaskInstance]:
        """
        Set the state of a TaskInstance and clear downstream tasks in failed or upstream_failed state.
        :param task_id: Task ID of the TaskInstance
        :param map_indexes: Only set TaskInstance if its map_index matches.
            If None (default), all mapped TaskInstances of the task are set.
        :param execution_date: Execution date of the TaskInstance
        :param run_id: The run_id of the TaskInstance
        :param state: State to set the TaskInstance to