Symfony\Component\ClassLoader\ApcClassLoader depreceated

since v4 of Symfony Symfony\Component\ClassLoader\ApcClassLoader is deprecated meaning the following code in settings.php throws an error

```
if ($settings['hash_salt']) {
$prefix = 'drupal.' . hash('sha256', 'drupal.' . $settings['hash_salt']);
$apc_loader = new \Symfony\Component\ClassLoader\ApcClassLoader($prefix, $class_loader);
unset($prefix);
$class_loader->unregister();
$apc_loader->register();
$class_loader = $apc_loader;
}
```

Not too familiar with ApcClassLoader to know what the alternative is.