**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
If the dag exists already, this flag will be ignored. If this optional parameter
        is not specified, the global config setting will be used.
    :type is_paused_upon_creation: bool or None
    :param jinja_environment_kwargs: additional configuration options to be passed to Jinja
        ``Environment`` for template rendering
        **Example**: to avoid Jinja from removing a trailing newline from template strings ::
            DAG(dag_id='my-dag',
                jinja_environment_kwargs={
                    'keep_trailing_newline': True,
                    # some other jinja2 Environment options here
                }
            )
        **See**: `Jinja Environment documentation
        <https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.Environment>`_
    :type jinja_environment_kwargs: dict
    :param tags: List of tags to help filtering DAGS in the UI.
    :type tags: List[str]
    """
    _comps = {
        'dag_id',
        'task_ids',
        'parent_dag',
        'start_date',
        'schedule_interval',
        'full_filepath',
        'template_searchpath',
        'last_loaded',
    }
    __serialized_fields: Optional[FrozenSet[str]] = None
    def __init__(