**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
ti2 = dr2.get_task_instance(task_id=op1.task_id, session=session)
        ti2.state = State.SCHEDULED
        session.commit()
        processor = mock.MagicMock()
        scheduler = SchedulerJob(num_runs=0)
        scheduler.processor_agent = processor
        scheduler.adopt_or_reset_orphaned_tasks()
        ti = dr.get_task_instance(task_id=op1.task_id, session=session)
        self.assertEqual(ti.state, State.NONE)
        ti2 = dr2.get_task_instance(task_id=op1.task_id, session=session)
        self.assertEqual(ti2.state, State.SCHEDULED, "Tasks run by Backfill Jobs should not be reset")
    @parameterized.expand([
        [State.UP_FOR_RETRY, State.FAILED],
        [State.QUEUED, State.NONE],
        [State.SCHEDULED, State.NONE],
        [State.UP_FOR_RESCHEDULE, State.NONE],
    ])
    def test_scheduler_loop_should_change_state_for_tis_without_dagrun(self,
                                                                       initial_task_state,
                                                                       expected_task_state):
        session = settings.Session()
        dag_id = 'test_execute_helper_should_change_state_for_tis_without_dagrun'
        dag = DAG(dag_id, start_date=DEFAULT_DATE, default_args={'owner': 'owner1'})
        with dag:
            op1 = DummyOperator(task_id='op1')
        
        with mock.patch.object(settings, "STORE_SERIALIZED_DAGS", True):
            dagbag = DagBag(dag_folder="/dev/null", include_examples=False)
            dagbag.bag_dag(dag, root_dag=dag)
            dagbag.sync_to_db()