**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
"""
        Returns the dag run for a given execution date if it exists, otherwise
        none.
        :param execution_date: The execution date of the DagRun to find.
        :param session:
        :return: The DagRun if found, otherwise None.
        """
        dagrun = (
            session.query(DagRun)
            .filter(DagRun.dag_id == self.dag_id, DagRun.execution_date == execution_date)
            .first()
        )
        return dagrun
    @provide_session
    def get_dagruns_between(self, start_date, end_date, session=None):