feat: optimize RCTKeyWindow() for iOS 15+ (#43066)

Summary:
This PR further optimizes RCTKeyWindow() for iOS 15+ removing the need for additional loop
bypass-github-export-checks
## Changelog:

[IOS] [ADDED] - optimize RCTKeyWindow() for iOS 15+

Pull Request resolved: #43066

Test Plan: Launch RNTester, check if proper keyWindow is returned for iOS 15+

Reviewed By: javache

Differential Revision: D54541838

Pulled By: cipolleschi

fbshipit-source-id: be79ff48f825d10c8fd71efc18629377aadc29fd

packages/react-native/React/Base/RCTUtils.m

@@ -569,6 +569,10 @@ BOOL RCTRunningInAppExtension(void)
     }
     UIWindowScene *windowScene = (UIWindowScene *)scene;
 
+    if (@available(iOS 15.0, *)) {
+      return windowScene.keyWindow;
+    }
+
     for (UIWindow *window in windowScene.windows) {
       if (window.isKeyWindow) {
         return window;