Add warning when app is running with the old architecture (#50735)

Summary:
Pull Request resolved: #50735

This change adds a warning in JS that is printed when the app is running using the old architecture.

The assumption is that, if it is running with Fabric, it is running with the new architecture. So running without Fabric implies old architecture.

## Changelog:
[General][Added] - Add warning when the app runs with the legacy architecture

Reviewed By: cortinico, rubennorte

Differential Revision: D73041156

Author: cipolleschi

packages/react-native/Libraries/ReactNative/renderApplication.js

@@ -104,5 +104,14 @@ export default function renderApplication<Props: Object>(
     useFabric: Boolean(fabric),
     useConcurrentRoot,
   });
+
+  const newArchitecture = !!fabric;
+  if (!newArchitecture) {
+    const warnOnce = require('../Utilities/warnOnce').default;
+    warnOnce(
+      '[OSS][OldArchDeprecatedWarning]',
+      'The app is running using the Legacy Architecture. The Legacy Architecture is deprecated and will be removed in a future version of React Native. Please consider migrating to the New Architecture. For more information, please see https://reactnative.dev/blog/2024/10/23/the-new-architecture-is-here',
+    );
+  }
   performanceLogger.stopTimespan('renderApplication_React_render');
 }