**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
cls = self.__class__
        result = cls.__new__(cls)
        memo[id(self)] = result
        for k, v in self.__dict__.items():
            if k not in ('user_defined_macros', 'user_defined_filters', 'params', '_log'):
                setattr(result, k, copy.deepcopy(v, memo))
        result.user_defined_macros = self.user_defined_macros
        result.user_defined_filters = self.user_defined_filters
        result.params = self.params
        if hasattr(self, '_log'):
            result._log = self._log
        return result
    def sub_dag(self, *args, **kwargs):