### Apache Airflow version

2.1.3

### Operating System

ContainerOS

### Versions of Apache Airflow Providers

apache-airflow-providers-amazon==2.1.0
apache-airflow-providers-celery==2.0.0
apache-airflow-providers-cncf-kubernetes==2.0.2
apache-airflow-providers-databricks==2.0.1
apache-airflow-providers-docker==2.1.0
apache-airflow-providers-elasticsearch==2.0.2
apache-airflow-providers-ftp==2.0.0
apache-airflow-providers-google==5.0.0
apache-airflow-providers-grpc==2.0.0
apache-airflow-providers-hashicorp==2.0.0
apache-airflow-providers-http==2.0.0
apache-airflow-providers-imap==2.0.0
apache-airflow-providers-microsoft-azure==3.1.0
apache-airflow-providers-mysql==2.1.0
apache-airflow-providers-papermill==2.0.1
apache-airflow-providers-postgres==2.0.0
apache-airflow-providers-redis==2.0.0
apache-airflow-providers-sendgrid==2.0.0
apache-airflow-providers-sftp==2.1.0
apache-airflow-providers-slack==4.0.0
apache-airflow-providers-sqlite==2.0.0
apache-airflow-providers-ssh==2.1.0

### Deployment

Official Apache Airflow Helm Chart

### Deployment details

Official Helm Chart on GKE using CeleryKubernetesExecutor.

### What happened

I'm unable to Run Tasks from the UI when using CeleryKubernetesExecutor. It just shows the error "Only works with the Celery or Kubernetes executors, sorry". 
<img width="1792" alt="Captura de Pantalla 2021-09-22 a la(s) 17 00 30" src="https://user-images.githubusercontent.com/2586758/134413668-5b638941-bb50-44ff-9349-d53326d1f489.png">




### What you expected to happen

It should be able to run Tasks from the UI when using CeleryKubernetesExecutor, as it only is a "selective superset" (based on queue) of both the Celery and Kubernetes executors.

### How to reproduce

1. Run an Airflow instance with the CeleryKubernetes executor.
2. Go to any DAG
3. Select a Task
4. Press the "Run" button.
5. Check the error.

### Anything else

I will append an example PR as a comment.

### Are you willing to submit PR?

- [x] 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/www/views.py
"""List providers."""
        providers_manager = ProvidersManager()
        providers = []
        for pi in providers_manager.providers.values():
            provider_info = pi[1]
            provider_data = {
                "package_name": provider_info["package-name"],
                "description": self._clean_description(provider_info["description"]),
                "version": pi[0],
                "documentation_url": get_doc_url_for_provider(provider_info["package-name"], pi[0]),
            }
            providers.append(provider_data)
        title = "Providers"
        doc_url = get_docs_url("apache-airflow-providers/index.html")
        return self.render_template(
            'airflow/providers.html',
            providers=providers,
            title=title,
            doc_url=doc_url,
        )
    def _clean_description(self, description):
        def _build_link(match_obj):
            text = match_obj.group(1)
            url = match_obj.group(2)
            return markupsafe.Markup(f'<a href="{url}">{text}</a>')
        cd = markupsafe.escape(description)
        cd = re.sub(r"`(.*)[\s+]+&lt;(.*)&gt;`__", _build_link, cd)
        cd = re.sub(r"\n", r"<br>", cd)
        return markupsafe.Markup(cd)
class PoolModelView(AirflowModelView):