**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
from tests.test_utils.db import (
    clear_db_dags, clear_db_errors, clear_db_jobs, clear_db_pools, clear_db_runs, clear_db_serialized_dags,
    clear_db_sla_miss, set_default_pool_slots,
)
from tests.test_utils.mock_executor import MockExecutor
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
PERF_DAGS_FOLDER = os.path.join(ROOT_FOLDER, "tests", "test_utils", "perf", "dags")
ELASTIC_DAG_FILE = os.path.join(PERF_DAGS_FOLDER, "elastic_dag.py")
TEST_DAG_FOLDER = os.environ['AIRFLOW__CORE__DAGS_FOLDER']
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
TRY_NUMBER = 1
PARSEABLE_DAG_FILE_CONTENTS = '"airflow DAG"'
UNPARSEABLE_DAG_FILE_CONTENTS = 'airflow DAG'
INVALID_DAG_WITH_DEPTH_FILE_CONTENTS = (
    "def something():\n"
    "    return airflow_DAG\n"
    "something()"
)
TEMP_DAG_FILENAME = "temp_dag.py"
@pytest.fixture(scope="class")
def disable_load_example():
    with conf_vars({('core', 'load_examples'): 'false'}):
        with env_vars({('core', 'load_examples'): 'false'}):
            yield
@pytest.mark.usefixtures("disable_load_example")
class TestDagFileProcessor(unittest.TestCase):
    @staticmethod
    def clean_db():