**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
:rtype: list[airflow.models.DagRun]
        """
        from airflow.models.dag import DagModel
        if max_number is None:
            max_number = cls.DEFAULT_DAGRUNS_TO_EXAMINE
        
        query = session.query(cls).filter(
            cls.state == State.RUNNING,
            cls.run_type != DagRunType.BACKFILL_JOB
        ).join(
            DagModel,
            DagModel.dag_id == cls.dag_id,
        ).filter(
            DagModel.is_paused.is_(False),
            DagModel.is_active.is_(True),
        ).order_by(
            nulls_first(cls.last_scheduling_decision, session=session),
            cls.execution_date,
        )
        if not settings.ALLOW_FUTURE_EXEC_DATES:
            query = query.filter(DagRun.execution_date <= func.now())
        return with_row_locks(query.limit(max_number), of=cls, **skip_locked(session=session))
    @staticmethod
    @provide_session
    def find(
        dag_id: Optional[Union[str, List[str]]] = None,
        run_id: Optional[str] = None,
        execution_date: Optional[datetime] = None,
        state: Optional[str] = None,
        external_trigger: Optional[bool] = None,