**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
scheduler._send_sla_callbacks_to_processor = mock.Mock()
        
        scheduler.dagbag.bag_dag(dag, root_dag=dag)
        scheduler.dagbag.sync_to_db()
        session = settings.Session()
        orm_dag = session.query(DagModel).get(dag.dag_id)
        assert orm_dag is not None
        
        scheduler._create_dag_runs([orm_dag], session)
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        ti = dr.get_task_instance('dummy')
        ti.set_state(state, session)
        scheduler._schedule_dag_run(dr, 0, session)
        expected_callback = DagCallbackRequest(
            full_filepath=dr.dag.fileloc,
            dag_id=dr.dag_id,
            is_failure_callback=bool(state == State.FAILED),
            execution_date=dr.execution_date,
            msg=expected_callback_msg
        )
        
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(expected_callback)
        
        
        scheduler._send_sla_callbacks_to_processor.assert_called_once_with(dag)
        session.rollback()
        session.close()
    def test_do_not_schedule_removed_task(self):