### 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
app = minimal_app_for_api
    create_user(
        app,  
        username="test",
        role_name="Test",
        permissions=[
            (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_USER),
            (permissions.ACTION_CAN_DELETE, permissions.RESOURCE_USER),
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_USER),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_USER),
        ],
    )
    create_user(app, username="test_no_permissions", role_name="TestNoPermissions")  
    yield app
    delete_user(app, username="test")  
    delete_user(app, username="test_no_permissions")  
class TestUserEndpoint: