**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
"""
        DagRuns with one successful and one future root task -> SUCCESS
        Noted: the DagRun state could be still in running state during CI.
        """
        clear_db_dags()
        dag_id = 'test_dagrun_states_root_future'
        dag = self.dagbag.get_dag(dag_id)
        dag.sync_to_db()
        scheduler = SchedulerJob(
            num_runs=1,
            executor=self.null_exec,
            subdir=dag.fileloc)
        scheduler.run()
        first_run = DagRun.find(dag_id=dag_id, execution_date=DEFAULT_DATE)[0]
        ti_ids = [(ti.task_id, ti.state) for ti in first_run.get_task_instances()]
        self.assertEqual(ti_ids, [('current', State.SUCCESS)])
        self.assertIn(first_run.state, [State.SUCCESS, State.RUNNING])
    def test_dagrun_deadlock_ignore_depends_on_past_advance_ex_date(self):