code: protected function regenerateSessionId()
  	{
  		@trigger_error('Using User::regenerateSessionId() has been deprecated and will no longer work in Contao 5.0. Use Symfony authentication instead.', E_USER_DEPRECATED);
  
  		$container = System::getContainer();
  		$strategy = $container->getParameter('security.authentication.session_strategy.strategy');
  
  		// Regenerate the session ID to harden against session fixation attacks
  		switch ($strategy)
  		{
  			case SessionAuthenticationStrategy::NONE:
  				break;
  
  			case SessionAuthenticationStrategy::MIGRATE:
  				$container->get('session')->migrate(); // do not destroy the old session
  				break;
  
  			case SessionAuthenticationStrategy::INVALIDATE:
  				$container->get('session')->invalidate();
  				break;
  
  			default:
  				throw new \RuntimeException(sprintf('Invalid session authentication strategy "%s"', $strategy));
  		}
  	}
docs: Regenerate the session ID
  
  @deprecated Deprecated since Contao 4.5, to be removed in Contao 5.0.
  Use Symfony authentication instead.