**Apache Airflow version**:
2.0.1

**Kubernetes version**:
Server Version: version.Info{Major:"1", Minor:"18+", GitVersion:"v1.18.9-eks-d1db3c", GitCommit:"d1db3c46e55f95d6a7d3e5578689371318f95ff9", GitTreeState:"clean", BuildDate:"2020-10-20T22:18:07Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"linux/amd64"}

**What happened**:
Clearing a failed subdag task with Downstream+Recursive does not automatically set the state of the parent dag to 'running' so that the downstream parent tasks can execute.

The work around is to manually set the state of the parent dag to running after clearing the subdag task

**What you expected to happen**:
With airflow version 1.10.4 the parent dag was automatically set to 'running' for this same scenario

**How to reproduce it**:
- Clear a failed subdag task selecting option for Downstream+Recursive
- See that all down stream tasks in the subdag as well as the parent dag have been cleared
- See that the parent dag is left in 'failed' state. 

airflow/models/dag.py
"""
        Get the next execution date after the given ``date_last_automated_dagrun``, according to
        schedule_interval, start_date, end_date etc.  This doesn't check max active run or any other
        "concurrency" type limits, it only performs calculations based on the various date and interval fields
        of this dag and it's tasks.
        :param date_last_automated_dagrun: The execution_date of the last scheduler or
            backfill triggered run for this dag
        :type date_last_automated_dagrun: pendulum.Pendulum
        """
        if not self.schedule_interval or self.is_subdag:
            return None
        
        if self.schedule_interval == '@once' and date_last_automated_dagrun:
            return None
        
        if not (self.catchup or self.schedule_interval == '@once'):
            
            
            
            now = timezone.utcnow()
            next_start = self.following_schedule(now)
            last_start = self.previous_schedule(now)
            if next_start <= now or isinstance(self.schedule_interval, timedelta):
                new_start = last_start
            else: