Code to be replaced with drupal_goto($url); :-

Write this use statement at top of the file-

use \Symfony\Component\HttpFoundation\RedirectResponse;

Rest code is to replace drupal_goto($url)

$response = new RedirectResponse($url); // $url = '<front>';
$response->send();

You need to replace the calls to drupal_goto().

Write this line at top of the file.

use \Symfony\Component\HttpFoundation\RedirectResponse;

Replace the call to drupal_goto() with the following code.

$response = new RedirectResponse($url); // $url = '<front>';
$response->send();