**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.

tests/sensors/test_external_task_sensor.py
#
#
#
import unittest
from datetime import time, timedelta
import pytest
from airflow import exceptions, settings
from airflow.exceptions import AirflowException, AirflowSensorTimeout
from airflow.models import DagBag, TaskInstance
from airflow.models.dag import DAG
from airflow.operators.bash import BashOperator
from airflow.operators.dummy import DummyOperator
from airflow.sensors.external_task import ExternalTaskMarker, ExternalTaskSensor
from airflow.sensors.time_sensor import TimeSensor
from airflow.serialization.serialized_objects import SerializedBaseOperator
from airflow.utils.state import State
from airflow.utils.timezone import datetime
DEFAULT_DATE = datetime(2015, 1, 1)
TEST_DAG_ID = 'unit_test_dag'
TEST_TASK_ID = 'time_sensor_check'
DEV_NULL = '/dev/null'
class TestExternalTaskSensor(unittest.TestCase):
    def setUp(self):