**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
no_backfills: bool = False,
        run_type: Optional[DagRunType] = None,
        session: Session = None,
        execution_start_date: Optional[datetime] = None,
        execution_end_date: Optional[datetime] = None
    ) -> List["DagRun"]:
        """
        Returns a set of dag runs for the given search criteria.
        :param dag_id: the dag_id or list of dag_id to find dag runs for
        :type dag_id: str or list[str]
        :param run_id: defines the run id for this dag run
        :type run_id: str
        :param run_type: type of DagRun
        :type run_type: airflow.utils.types.DagRunType
        :param execution_date: the execution date
        :type execution_date: datetime.datetime or list[datetime.datetime]
        :param state: the state of the dag run
        :type state: str
        :param external_trigger: whether this dag run is externally triggered
        :type external_trigger: bool
        :param no_backfills: return no backfills (True), return all (False).
            Defaults to False
        :type no_backfills: bool
        :param session: database session
        :type session: sqlalchemy.orm.session.Session
        :param execution_start_date: dag run that was executed from this date
        :type execution_start_date: datetime.datetime
        :param execution_end_date: dag run that was executed until this date
        :type execution_end_date: datetime.datetime
        """