### Apache Airflow version

2.1.3 (latest released)

### Operating System

CentOS Stream release 8

### Versions of Apache Airflow Providers

None of them are relevant

### Deployment

Virtualenv installation

### Deployment details

mkdir /srv/airflow
cd /srv/airflow
virtualenv venv
source venv/bin/activate
pip install apache-airflow==2.1.3
AIRFLOW_HOME and AIRFLOW_CONFIG is specified via environment variables in /etc/sysconfig/airflow, which is in turn used as EnvironmentFile in systemd service files.
systemctl start airflow-{scheduler,webserver,kerberos}
Python version: 3.9.2
LocalExecutors are used

### What happened

On the Web UI, I had cleared failed tasks, which have been cleared properly, but the DagRun became black with a literal value of "DagRunState.QUEUED", therefore it can't be scheduled again.

### What you expected to happen

DagRun state should be 'queued'.

### How to reproduce

Just clear any tasks on the Web UI. I wonder how could it be that nobody noticed this issue.

### Anything else

Here's a patch to fix it. Maybe the __str__ method should be different, or the database/persistence layer should handle this, but for now, this solves the problem.

```patch
--- airflow/models/dag.py.orig  2021-08-28 09:48:05.465542450 +0200
+++ airflow/models/dag.py       2021-08-28 09:47:34.272639960 +0200
@@ -1153,7 +1153,7 @@
         confirm_prompt=False,
         include_subdags=True,
         include_parentdag=True,
-        dag_run_state: DagRunState = DagRunState.QUEUED,
+        dag_run_state: DagRunState = DagRunState.QUEUED.value,
         dry_run=False,
         session=None,
         get_tis=False,
@@ -1369,7 +1369,7 @@
         confirm_prompt=False,
         include_subdags=True,
         include_parentdag=False,
-        dag_run_state=DagRunState.QUEUED,
+        dag_run_state=DagRunState.QUEUED.value,
         dry_run=False,
     ):
```

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/utils/state.py
#
#
#
from enum import Enum
from typing import Dict, FrozenSet, Tuple
from airflow.settings import STATE_COLORS
from airflow.utils.types import Optional
class TaskInstanceState(str, Enum):
    """
    Enum that represents all possible states that a Task Instance can be in.
    Note that None is also allowed, so always use this in a type hint with Optional.
    """
    
    
    REMOVED = "removed"  
    SCHEDULED = "scheduled"  
    
    QUEUED = "queued"  
    RUNNING = "running"  
    SUCCESS = "success"  
    SHUTDOWN = "shutdown"  
    RESTARTING = "restarting"  
    FAILED = "failed"  
    UP_FOR_RETRY = "up_for_retry"  
    UP_FOR_RESCHEDULE = "up_for_reschedule"  
    UPSTREAM_FAILED = "upstream_failed"  
    SKIPPED = "skipped"  
    SENSING = "sensing"  
    DEFERRED = "deferred"  
    def __str__(self) -> str: