Add useModernStrictMode as dynamic flag on www (#28346)

## Summary

Preparing modern strict mode rollout with dynamic feature flag

## How did you test this change?

![Screenshot 2024-02-15 at 10 09
49 AM](https://github.com/facebook/react/assets/8965173/9e90efc2-3578-4e63-ae2c-63d4a4e194b3)

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -28,6 +28,7 @@ export const alwaysThrottleRetries = __VARIANT__;
 export const enableDO_NOT_USE_disableStrictPassiveEffect = __VARIANT__;
 export const enableUseDeferredValueInitialArg = __VARIANT__;
 export const enableRenderableContext = __VARIANT__;
+export const useModernStrictMode = __VARIANT__;
 
 export const enableRetryLaneExpiration = __VARIANT__;
 export const retryLaneExpirationMs = 5000;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -36,6 +36,7 @@ export const {
   transitionLaneExpirationMs,
   enableInfiniteRenderLoopDetection,
   enableRenderableContext,
+  useModernStrictMode,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.
@@ -103,7 +104,6 @@ export const allowConcurrentByDefault = true;
 
 export const consoleManagedByDevToolsDuringStrictMode = true;
 
-export const useModernStrictMode = false;
 export const enableFizzExternalRuntime = true;
 
 export const forceConcurrentByDefaultForTesting = false;