Avoid use of the $_GET super global

Removed the GET super global uses and opened an MR. Since the request_stack already has a blueprint to be injected in FormBase (see code below), which is extended by SubscriberExportForm.php, I used that to implement the injection.

protected function getRequest() {
    if (!$this->requestStack) {
      $this->requestStack = \Drupal::service('request_stack');
    }
    return $this->requestStack->getCurrentRequest();
  }

And since we cannot use isset() on the result of an expression, I needed to assign the request to a variable first so then we could keep the current logic, like:

$states = $this->getRequest()->query->get('states');
$default['states'] = isset($states) ? $states : ['active' => 'active'];

Thank you kindly for the code review @Berdir. Assigning myself to implement and test the proposed changes.

I made a commit implementing the changes that @Berdir proposed in the MR, which resulted in a much cleaner fix.

Thanks for the review @AdamPS! Made a commit addressing what you suggested in #6.