### 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
return self.value
class DagRunState(str, Enum):
    """
    Enum that represents all possible states that a DagRun can be in.
    These are "shared" with TaskInstanceState in some parts of the code,
    so please ensure that their values always match the ones with the
    same name in TaskInstanceState.
    """
    QUEUED = "queued"
    RUNNING = "running"
    SUCCESS = "success"
    FAILED = "failed"
class State: