fix: stale RCTModalHostViewController blocks touch inputs

[mdjastrzebski]

Summary:

Resolves #54856

Fixes a but when RCTModalHostViewController tries to dismiss itself using the following code:

- (void)dismissViewController:(UIViewController *)modalViewController
                     animated:(BOOL)animated
                   completion:(void (^)(void))completion
{
  [modalViewController dismissViewControllerAnimated:animated completion:completion];
}

But since it's already presenting another VC on top of it (e.g., login web view VC, photo picker VC, etc), it dismisses that view but not itself, which results in a transparent modal VC (all the UI is unmounted) that is the top VC blocking all touch input.

The change is minimal: instead of calling dismissViewControllerAnimated:completion: on modalViewController, which has two different behaviors depending on whether that VC is presenting another VC, it calls it on modalViewController.presentingViewController, which has a unique behavior of dismissing modalViewController.

Changelog:

Test Plan:

1. Run Modal examples in the RN Tester app to confirm that this code still works ok in the base case.
2. Run the reproducer from [iOS] ModalHostViewController not properly dismissed if presenting another VC #54856 with and without the change to confirm this code fixes the issue.

[mdjastrzebski]

Danger change is failing due to non-related error: https://github.com/facebook/react-native/actions/runs/20138652296/job/57799224046