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

Summary:
Pull Request resolved: facebook#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

fbshipit-source-id: 89a14f6370ae54b9d115e0ef672f29084d009a8e

Author: cipolleschi

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

@@ -13,6 +13,7 @@ import type {IPerformanceLogger} from '../Utilities/createPerformanceLogger';
 
 import GlobalPerformanceLogger from '../Utilities/GlobalPerformanceLogger';
 import PerformanceLoggerContext from '../Utilities/PerformanceLoggerContext';
+import warnOnce from '../Utilities/warnOnce';
 import AppContainer from './AppContainer';
 import DisplayMode, {type DisplayModeType} from './DisplayMode';
 import getCachedComponentWithDebugName from './getCachedComponentWithDebugName';
@@ -104,5 +105,13 @@ export default function renderApplication<Props: Object>(
     useFabric: Boolean(fabric),
     useConcurrentRoot,
   });
+
+  const newArchitecture = !!fabric;
+  if (!newArchitecture) {
+    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');
 }