**Apache Airflow version**:

2.0.1

**What happened**:

When clearing task across dags using `ExternalTaskMarker` the dag state of the external `DagRun` is not set to active. So cleared tasks in the external dag will not automatically start if the `DagRun` is a Failed or Succeeded state.

**What you expected to happen**:

The external `DagRun` run should also be set to Running state.

**How to reproduce it**:

Clear tasks in an external dag using an ExternalTaskMarker.


**Anything else we need to know**:

Looking at the code is has:

https://github.com/apache/airflow/blob/b23fc137812f5eabf7834e07e032915e2a504c17/airflow/models/dag.py#L1323-L1335

It seems like it intentionally calls the dag member method `set_dag_run_state` instead of letting the helper function `clear_task_insntances` set the `DagRun` state. But the member method will only change the state of `DagRun`s of dag where the original task is, while I believe `clear_task_instances` would correctly change the state of all involved `DagRun`s.

airflow/models/taskinstance.py
pool: Optional[str] = None,
        session=None,
    ) -> bool:
        """
        Checks dependencies and then sets state to RUNNING if they are met. Returns
        True if and only if state is set to RUNNING, which implies that task should be
        executed, in preparation for _run_raw_task
        :param verbose: whether to turn on more verbose logging
        :type verbose: bool
        :param ignore_all_deps: Ignore all of the non-critical dependencies, just runs
        :type ignore_all_deps: bool
        :param ignore_depends_on_past: Ignore depends_on_past DAG attribute
        :type ignore_depends_on_past: bool
        :param ignore_task_deps: Don't check the dependencies of this TaskInstance's task
        :type ignore_task_deps: bool
        :param ignore_ti_state: Disregards previous task instance state
        :type ignore_ti_state: bool
        :param mark_success: Don't run the task, mark its state as success
        :type mark_success: bool
        :param test_mode: Doesn't record success or failure in the DB
        :type test_mode: bool
        :param job_id: Job (BackfillJob / LocalTaskJob / SchedulerJob) ID
        :type job_id: str
        :param pool: specifies the pool to use to run the task instance
        :type pool: str
        :param session: SQLAlchemy ORM Session
        :type session: Session
        :return: whether the state was changed to running or not
        :rtype: bool
        """