**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
test_mode=test_mode,
            job_id=job_id,
            pool=pool,
            session=session,
        )
        if not res:
            return
        try:
            error_fd = NamedTemporaryFile(delete=True)
            self._run_raw_task(
                mark_success=mark_success,
                test_mode=test_mode,
                job_id=job_id,
                pool=pool,
                error_file=error_fd.name,
                session=session,
            )
        finally:
            error = None if self.state == State.SUCCESS else load_error_file(error_fd)
            error_fd.close()
            self._run_finished_callback(error=error)
    def dry_run(self):
        """Only Renders Templates for the TI"""
        task = self.task
        task_copy = task.prepare_for_execution()
        self.task = task_copy
        self.render_templates()
        task_copy.dry_run()
    @provide_session
    def _handle_reschedule(self, actual_start_date, reschedule_exception, test_mode=False, session=None):