**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/dag.py
from airflow.decorators import task
        return functools.partial(task, dag=self)
    def add_task(self, task):
        """
        Add a task to the DAG
        :param task: the task you want to add
        :type task: task
        """
        if not self.start_date and not task.start_date:
            raise AirflowException("Task is missing the start_date parameter")
        
        elif not task.start_date:
            task.start_date = self.start_date
        
        
        elif self.start_date:
            task.start_date = max(task.start_date, self.start_date)
        
        if not task.end_date:
            task.end_date = self.end_date
        
        
        elif task.end_date and self.end_date:
            task.end_date = min(task.end_date, self.end_date)
        if (
            task.task_id in self.task_dict and self.task_dict[task.task_id] is not task
        ) or task.task_id in self._task_group.used_group_ids: