Move modern strict to experimental

Author: rickhanlonii

packages/react/src/__tests__/ReactStrictMode-test.js

@@ -123,6 +123,8 @@ describe('ReactStrictMode', () => {
       'render',
       'render',
       'componentDidMount',
+      'componentWillUnmount',
+      'componentDidMount',
     ]);
 
     log = [];

packages/shared/ReactFeatureFlags.js

@@ -147,7 +147,7 @@ export const disableJavaScriptURLs = false;
 // Modern <StrictMode /> behaviour aligns more with what components
 // components will encounter in production, especially when used With <Offscreen />.
 // TODO: clean up legacy <StrictMode /> once tests pass WWW.
-export const useModernStrictMode = false;
+export const useModernStrictMode = __NEXT_MAJOR__;
 
 // Not ready to break experimental yet.
 // Remove IE and MsApp specific workarounds for innerHTML