**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
"""Get the email subject content for exceptions."""
        
        
        use_default = not hasattr(self, 'task')
        exception_html = str(exception).replace('\n', '<br>')
        default_subject = 'Airflow alert: {{ti}}'
        
        
        
        default_html_content = (
            'Try {{try_number}} out of {{max_tries + 1}}<br>'
            'Exception:<br>{{exception_html}}<br>'
            'Log: <a href="{{ti.log_url}}">Link</a><br>'
            'Host: {{ti.hostname}}<br>'
            'Mark success: <a href="{{ti.mark_success_url}}">Link</a><br>'
        )
        default_html_content_err = (
            'Try {{try_number}} out of {{max_tries + 1}}<br>'
            'Exception:<br>Failed attempt to attach error logs<br>'
            'Log: <a href="{{ti.log_url}}">Link</a><br>'
            'Host: {{ti.hostname}}<br>'
            'Mark success: <a href="{{ti.mark_success_url}}">Link</a><br>'
        )