**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.
airflow/models/dagrun.py
ti.state = State.REMOVED
            should_restore_task = (task is not None) and ti.state == State.REMOVED
            if should_restore_task:
                self.log.info("Restoring task '%s' which was previously "
                              "removed from DAG '%s'", ti, dag)
                Stats.incr("task_restored_to_dag.{}".format(dag.dag_id), 1, 1)
                ti.state = State.NONE
            session.merge(ti)
        
        for task in dag.task_dict.values():
            if task.start_date > self.execution_date and not self.is_backfill:
                continue
            if task.task_id not in task_ids:
                Stats.incr(
                    "task_instance_created-{}".format(task.task_type),
                    1, 1)
                ti = TI(task, self.execution_date)
                task_instance_mutation_hook(ti)
                session.add(ti)
        try:
            session.flush()
        except IntegrityError as err:
            self.log.info(str(err))
            self.log.info('Hit IntegrityError while creating the TIs for '
                          f'{dag.dag_id} - {self.execution_date}.')
            self.log.info('Doing session rollback.')
            
            session.rollback()
    @staticmethod
    def get_run(session: Session, dag_id: str, execution_date: datetime):