**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
:param only_running: Only clear running tasks.
        :type only_running: bool
        :param confirm_prompt: Ask for confirmation
        :type confirm_prompt: bool
        :param include_subdags: Clear tasks in subdags and clear external tasks
            indicated by ExternalTaskMarker
        :type include_subdags: bool
        :param include_parentdag: Clear tasks in the parent dag of the subdag.
        :type include_parentdag: bool
        :param dag_run_state: state to set DagRun to
        :param dry_run: Find the tasks to clear but don't clear them.
        :type dry_run: bool
        :param session: The sqlalchemy session to use
        :type session: sqlalchemy.orm.session.Session
        :param get_tis: Return the sqlalchemy query for finding the TaskInstance without clearing the tasks
        :type get_tis: bool
        :param recursion_depth: The recursion depth of nested calls to DAG.clear().
        :type recursion_depth: int
        :param max_recursion_depth: The maximum recursion depth allowed. This is determined by the
            first encountered ExternalTaskMarker. Default is None indicating no ExternalTaskMarker
            has been encountered.
        :type max_recursion_depth: int
        :param dag_bag: The DagBag used to find the dags
        :type dag_bag: airflow.models.dagbag.DagBag
        :param visited_external_tis: A set used internally to keep track of the visited TaskInstance when
            clearing tasks across multiple DAGs linked by ExternalTaskMarker to avoid redundant work.
        :type visited_external_tis: set
        """
        TI = TaskInstance
        tis = session.query(TI)