**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.
tests/jobs/test_scheduler_job.py
orm_dag = DagModel(dag_id=dag.dag_id)
            session.merge(orm_dag)
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        scheduler = SchedulerJob()
        scheduler.processor_agent = mock.MagicMock()
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        dag.clear()
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        assert dr is not None
        with create_session() as session:
            ti = dr.get_task_instances(session=session)[0]
            ti.state = state
            ti.start_date = start_date
            ti.end_date = end_date
            count = scheduler._schedule_dag_run(dr, 0, session)
            assert count == 1
            session.refresh(ti)
            assert ti.state == State.SCHEDULED
    @parameterized.expand([
        [State.NONE, None, None],
        [State.UP_FOR_RETRY, timezone.utcnow() - datetime.timedelta(minutes=30),
         timezone.utcnow() - datetime.timedelta(minutes=15)],
        [State.UP_FOR_RESCHEDULE, timezone.utcnow() - datetime.timedelta(minutes=30),
         timezone.utcnow() - datetime.timedelta(minutes=15)],
    ])
    def test_dag_file_processor_process_task_instances_depends_on_past(self, state, start_date, end_date):