Use Session service instead of $_SESSION

Problem/Motivation

The current code is incompatible with custom session handlers (redis sessions for example) because it directly accesses the $_SESSION super global variable.
cf. https://git.drupalcode.org/project/keycloak/-/blob/2.2.x/src/Controller/KeycloakController.php?ref_type=heads#L124

  /**
   * Login the user using the Keycloak openid_connect client.
   */
  public function login() {
    $this->session->saveDestination();
    $client_name = 'keycloak';

    $configuration = $this->config('openid_connect.settings.keycloak')->get('settings');
    $client = $this->pluginManager->createInstance(
      $client_name,
      $configuration
    );
    $scopes = $this->claims->getScopes();
    $_SESSION['openid_connect_op'] = 'login';
    return $client->authorize($scopes);
  }

Steps to reproduce

1. Use a custom session manager service
2. Try to authenticate using Keycloak, it will fail silently.

Proposed resolution

Use the right service (already injected), from OpenID module :

-     $_SESSION['openid_connect_op'] = 'login';
+     $this->session->saveOp('login');

Patch tested on Drupal 10.2.3, PHP 8.1, 8.2