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)

DiffTrain build for commit https://github.com/facebook/react/commit/ef72271c2d1234c9d1e1358f8083021089a50faa.

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react-test-renderer/cjs/ReactTestRenderer-dev.js

@@ -25633,7 +25633,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "18.3.0-canary-a9cc32511-20240215";
+    var ReactVersion = "18.3.0-canary-ef72271c2-20240216";
 
     // Might add PROFILE later.
 

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react-test-renderer/cjs/ReactTestRenderer-prod.js

@@ -9179,7 +9179,7 @@ var devToolsConfig$jscomp$inline_1018 = {
     throw Error("TestRenderer does not support findFiberByHostInstance()");
   },
   bundleType: 0,
-  version: "18.3.0-canary-a9cc32511-20240215",
+  version: "18.3.0-canary-ef72271c2-20240216",
   rendererPackageName: "react-test-renderer"
 };
 var internals$jscomp$inline_1199 = {
@@ -9210,7 +9210,7 @@ var internals$jscomp$inline_1199 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "18.3.0-canary-a9cc32511-20240215"
+  reconcilerVersion: "18.3.0-canary-ef72271c2-20240216"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_1200 = __REACT_DEVTOOLS_GLOBAL_HOOK__;

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react-test-renderer/cjs/ReactTestRenderer-profiling.js

@@ -9607,7 +9607,7 @@ var devToolsConfig$jscomp$inline_1060 = {
     throw Error("TestRenderer does not support findFiberByHostInstance()");
   },
   bundleType: 0,
-  version: "18.3.0-canary-a9cc32511-20240215",
+  version: "18.3.0-canary-ef72271c2-20240216",
   rendererPackageName: "react-test-renderer"
 };
 var internals$jscomp$inline_1240 = {
@@ -9638,7 +9638,7 @@ var internals$jscomp$inline_1240 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "18.3.0-canary-a9cc32511-20240215"
+  reconcilerVersion: "18.3.0-canary-ef72271c2-20240216"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_1241 = __REACT_DEVTOOLS_GLOBAL_HOOK__;

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/cjs/React-dev.js

@@ -24,7 +24,7 @@ if (__DEV__) {
     ) {
       __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
     }
-    var ReactVersion = "18.3.0-canary-a9cc32511-20240215";
+    var ReactVersion = "18.3.0-canary-ef72271c2-20240216";
 
     // ATTENTION
     // When adding new symbols to this file,

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/cjs/React-prod.js

@@ -590,4 +590,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactCurrentDispatcher.current.useTransition();
 };
-exports.version = "18.3.0-canary-a9cc32511-20240215";
+exports.version = "18.3.0-canary-ef72271c2-20240216";

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/cjs/React-profiling.js

@@ -586,7 +586,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactCurrentDispatcher.current.useTransition();
 };
-exports.version = "18.3.0-canary-a9cc32511-20240215";
+exports.version = "18.3.0-canary-ef72271c2-20240216";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled-rn/facebook-fbsource/xplat/js/react-native-github/Libraries/Renderer/REVISION

@@ -1 +1 @@
-a9cc32511a12c261ee719e5383818182800d6af4
+ef72271c2d1234c9d1e1358f8083021089a50faa