= Legacy Promise API removed from Keycloak JS adapter

The legacy Promise API methods have been removed from the Keycloak JS adapter. This means that calling `.success()` and `.error()` on promises returned from the adapter is no longer possible. Instead standardized Promise methods such as https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then[`.then()`] and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch[`.catch()`] should be used.

*Before:*
```javascript
const keycloak = new Keycloak();

keycloak.init()
  .success(function(authenticated) {
    alert(authenticated ? 'authenticated' : 'not authenticated');
  }).error(function() {
    alert('failed to initialize');
  });
```

*After:*
```javascript
const keycloak = new Keycloak();

keycloak.init()
  .then(function(authenticated) {
    alert(authenticated ? 'authenticated' : 'not authenticated');
  }).catch(function() {
    alert('failed to initialize');
  });
```

Or alternatively, when using the https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await[`await`] keyword to unwrap these promises:

```javascript
const keycloak = new Keycloak();

try {
  const authenticated = await keycloak.init();
  alert(authenticated ? 'authenticated' : 'not authenticated');
} catch (error) {
  alert('failed to initialize');
}
```
