Remove menu active trail service from SystemMenuBlock

The menu active trail service has been removed from SystemMenuBlock.

If you maintain a custom or contributed module that has a block plugin that extends \Drupal\system\Plugin\Block\SystemMenuBlock you might need to update it. If the plugin is using the service or the $menuActiveTrail property you will need to make changes. Ideally code should not extend from concrete plugin classes as these are internal as per https://www.drupal.org/core/d8-bc-policy but it is possible to make the code more robust by removing any constructor overrides and overriding the ::create() method in your plugin. For example:

  /**
   * The active menu trail service.
   *
   * @var \Drupal\Core\Menu\MenuActiveTrailInterface
   */
  protected $menuActiveTrail;

  /**
   * {@inheritdoc}
   */
  public static function create($config, $plugin_id, $def, $container) {
    $static = parent::create($config, $plugin_id, $def, $container);
    $static->setMenuActiveTrail($container->get('menu.active_trail'));
    return $static;
  }

  /**
   * Sets the menu active trail service.
   *
   * @param \Drupal\Core\Menu\MenuActiveTrailInterface $menu_active_trail
   *   The menu active trail service.
   */
  protected function setMenuActiveTrail(MenuActiveTrailInterface $menu_active_trail) {
    $this->menuActiveTrail = $menu_active_trail;
  }