Sign-Up email is not sent to admin for registration pending approval

### Bug description

Upon user sign-up no email to admin(s) is sent.

#### Expected behaviour

Admin(s) should get an email about user's Sign-Up attempt.

Any strategy to fix this:

- a hint on how to properly configure the notification email to admin(s)
- a hint on how to patch the NativeAuthenticator code, e.g. similar to the self-serve approval way [#L343](https://github.com/jupyterhub/nativeauthenticator/blob/main/nativeauthenticator/nativeauthenticator.py#L343)

#### Actual behaviour

Upon user sign-up a message [#Line125](https://github.com/jupyterhub/nativeauthenticator/blob/main/nativeauthenticator/handlers.py#L125)

`Your information has been sent to the admin.`

is being displayed, but no email to admin is triggered to localhost Postfix email server.

### How to reproduce

1. Just 'Sign-Up'
2. Observe the `Your information has been sent to the admin.`
3. Assume the admin actually got the email
4. Wait indefinitely till the admin checks your request in the JH Authorization area page

### Your personal set up

- OS: Ubuntu

- Version(s): ubuntu 22.04

Full environment

```
# paste output of `pip freeze` or `conda list` here
```

Configuration

```python
# jupyterhub_config.py
```

Logs

```
# paste relevant logs here, if any
```