**Apache Airflow version**: 2.0.0a1

**What happened**:

On my dev environment I have a lot of DAG runs with failed tasks, where the *downstream* task instances then have been set to `upstream_failed`. The scheduler is now refusing to schedule more instances for such DAGs.

The scheduler looks for such task instances that are not in the `State.finished` set:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/jobs/scheduler_job.py#L1482-L1488

However, `State.finished` is currently not a proper inverse of *running* states, it only includes success, failed, and skipped:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/utils/state.py#L110-L114

What is missing is the task instance `upstream_failed` state, so currently my log is being filled (very rapidly) with:

```
[2020-10-18 13:22:18,503] {{scheduler_job.py:1658}} INFO - DAG test_dag already has 26 active runs, not queuing any more tasks
```

Those 26 'active runs' are simply dag runs that failed and have 'upstream_failed' task instances.

Either `upstream_failed` should be added to `State.finished`, or the Scheduler task instance test should explicitly take that state into account.
airflow/models/dagrun.py
"""The previous DagRun, if there is one"""
        filters = [
            DagRun.dag_id == self.dag_id,
            DagRun.execution_date < self.execution_date,
        ]
        if state is not None:
            filters.append(DagRun.state == state)
        return session.query(DagRun).filter(
            *filters
        ).order_by(
            DagRun.execution_date.desc()
        ).first()
    @provide_session
    def get_previous_scheduled_dagrun(self, session: Session = None):
        """The previous, SCHEDULED DagRun, if there is one"""
        dag = self.get_dag()
        return session.query(DagRun).filter(
            DagRun.dag_id == self.dag_id,
            DagRun.execution_date == dag.previous_schedule(self.execution_date)
        ).first()
    @provide_session
    def update_state(
        self,
        session: Session = None,
        execute_callbacks: bool = True
    ) -> Tuple[List[TI], Optional[callback_requests.DagCallbackRequest]]:
        """
        Determines the overall state of the DagRun based on the state
        of its TaskInstances.
        :param session: Sqlalchemy ORM Session