Get rid of deprecated code (breaking D8 compatibility)

* GetResponseForExceptionEvent was already replaced by Symfony\Component\HttpKernel\Event\ExceptionEvent in #3289460: Automated Drupal 10 compatibility fixes → (v8.x-3.6). It is introduced since Symfony 4.4 which (AFAICT) means that all Drupal >= 9.2 installations always have it. So there should be no compatibility issues.

* TODO about EmailValidatorInterface cleaned up in this issue. (No backward compatibility issues; this change just provides forward compatibility with theoretical future Core/custom application versions that change EmailValidatorInterface implementations.)

* SamlTest was done in #3304216: Move tests from Simpletest/WebTestBase to PHPUnit/BrowserTestBase → (v8.x-3.6).

Automatically closed - issue fixed for 2 weeks with no activity.

Uploading a patch file just to provide a convenient mechanism for folks like us to pull this change into 8.x-3.x ahead of a release being cut.

Thanks for the work on this :-)