RedirectLeadingSlashesSubscriber renamed to the RedirectSuccessiveSlashesSubscriber

This change is a follow-up to #2852361: Ignore repeated slashes in the incoming path like Drupal <= 7
The service was modified to handle multiple successive slashes in the URL, so the current name does not reflect what the service does.
Service and class names were changed.

Old class usage:

use \Drupal\Core\EventSubscriber\RedirectLeadingSlashesSubscriber;

New class usage:

use \Drupal\Core\EventSubscriber\RedirectSuccessiveSlashesSubscriber;

Old service use:

\Drupal::service('redirect_leading_slashes_subscriber');

New service use:

\Drupal::service('redirect_successive_slashes_subscriber');