**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
clear_db_runs()
        clear_db_pools()
        clear_db_dags()
        clear_db_sla_miss()
        clear_db_errors()
        
        
        self.null_exec = MockExecutor()
    @classmethod
    @patch("airflow.models.dagbag.settings.STORE_SERIALIZED_DAGS", True)
    def setUpClass(cls):
        
        non_serialized_dagbag = DagBag(read_dags_from_db=False, include_examples=False)
        non_serialized_dagbag.sync_to_db()
        cls.dagbag = DagBag(read_dags_from_db=True)
    def test_is_alive(self):
        job = SchedulerJob(None, heartrate=10, state=State.RUNNING)
        self.assertTrue(job.is_alive())
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=20)
        self.assertTrue(job.is_alive())
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=31)
        self.assertFalse(job.is_alive())
        
        
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(days=1)
        self.assertFalse(job.is_alive())
        job.state = State.SUCCESS
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=10)
        self.assertFalse(job.is_alive(), "Completed jobs even with recent heartbeat should not be alive")
    def run_single_scheduler_loop_with_no_dags(self, dags_folder):