Fix default modalPresentationStyle on < iOS 13 (#6494)

Changes the default from overFullScreen to fullScreen on iOS 12 and below.

According to #5709
> Below iOS 13, the default presentation style is always fullScreen presentation style.

Also looking at the blame it was indeed "fullScreen" not "overFullScreen"

This is also inline with the docs https://wix.github.io/react-native-navigation/api/options-root#modalpresentationstyle

> The default presentation style is different on each platform.
> iOS 12 and below - fullScreen
> iOS 13 and above - pageSheet

Co-authored-by: Yogev Ben David <yogev132@gmail.com>
Co-authored-by: Guy Carmeli <guyca@users.noreply.github.com>

lib/ios/RNNConvert.m

@@ -6,7 +6,7 @@ + (UIModalPresentationStyle)defaultModalPresentationStyle {
     if (@available(iOS 13.0, *)) {
         return UIModalPresentationAutomatic;
     } else {
-        return UIModalPresentationOverFullScreen;
+        return UIModalPresentationFullScreen;
     }
 }