### Apache Airflow version

2.2.0b2 (beta snapshot)

### Operating System

Debian buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Astronomer

### Deployment details

Astro dev start

### What happened

Unable to change username while updating user information through API though it's possible from UI.

### What you expected to happen

Either username should not be editable from UI or It should be editable from API.

### How to reproduce

1. Use a patch request with the below URL.
`{{url}}/users/{{username}}`
2. In payload use a different username
```
 {
        "username": "{{username1}}",
        "password": "password1",
        "email": "{{email}}@example.com",
        "first_name": "{{$randomFirstName}}",
        "last_name": "{{$randomLastName}}",
        "roles":[{ "name": "Op"}]
}
```


### Anything else

_No response_

### 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)

tests/api_connexion/endpoints/test_user_endpoint.py
@pytest.fixture(autouse=True)
    def setup_attrs(self, configured_app) -> None:
        self.app = configured_app
        self.client = self.app.test_client()  
        self.session = self.app.appbuilder.get_session
    def teardown_method(self) -> None:
        
        users = self.session.query(User).filter(User.changed_on == timezone.parse(DEFAULT_TIME))
        users.delete(synchronize_session=False)
        self.session.commit()
    def _create_users(self, count, roles=None):
        
        
        if roles is None:
            roles = []
        return [
            User(
                first_name=f'test{i}',
                last_name=f'test{i}',
                username=f'TEST_USER{i}',
                email=f'mytest@test{i}.org',
                roles=roles or [],
                created_on=timezone.parse(DEFAULT_TIME),
                changed_on=timezone.parse(DEFAULT_TIME),
            )
            for i in range(1, count + 1)
        ]
class TestGetUser(TestUserEndpoint):