**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/taskinstance.py
backoff, retry_delay is used as base and will be converted to seconds.
        """
        delay = self.task.retry_delay
        if self.task.retry_exponential_backoff:
            
            
            
            min_backoff = int(math.ceil(delay.total_seconds() * (2 ** (self.try_number - 2))))
            
            ti_hash = int(
                hashlib.sha1(
                    "{}#{}#{}#{}".format(
                        self.dag_id, self.task_id, self.execution_date, self.try_number  
                    ).encode('utf-8')
                ).hexdigest(),
                16,
            )
            
            modded_hash = min_backoff + ti_hash % min_backoff
            
            
            
            
            
            delay_backoff_in_seconds = min(modded_hash, timedelta.max.total_seconds() - 1)
            delay = timedelta(seconds=delay_backoff_in_seconds)
            if self.task.max_retry_delay:
                delay = min(self.task.max_retry_delay, delay)
        return self.end_date + delay
    def ready_for_retry(self):