Move modern strict to experimental (facebook#28152)

Turn this on 

Edited: ope, nvm
<details>
Looks like there's still an outstanding issue with this. The original PR
turned off a strict effects test, which causes a stray
`componentWillUnmount` to fire.


facebook@5d1ce65#diff-19df471970763c4790c2cc0811fd2726cc6a891b0e1d5dedbf6d0599240c127aR70


Before:
```js
expect(log).toEqual([
      'constructor',
      'constructor',
      'getDerivedStateFromProps',
      'getDerivedStateFromProps',
      'render',
      'render',
      'componentDidMount',
    ]);
```

After:

```js
expect(log).toEqual([
      'constructor',
      'constructor',
      'getDerivedStateFromProps',
      'getDerivedStateFromProps',
      'render',
      'render',
      'componentDidMount',
      'componentWillUnmount',
      'componentDidMount',
    ]);
```

So there's a bug somewhere
</details>

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -3792,12 +3792,9 @@ function commitDoubleInvokeEffectsInDEV(
   hasPassiveEffects: boolean,
 ) {
   if (__DEV__) {
-    if (useModernStrictMode) {
+    if (useModernStrictMode && root.tag !== LegacyRoot) {
       let doubleInvokeEffects = true;
 
-      if (root.tag === LegacyRoot && !(root.current.mode & StrictLegacyMode)) {
-        doubleInvokeEffects = false;
-      }
       if (
         root.tag === ConcurrentRoot &&
         !(root.current.mode & (StrictLegacyMode | StrictEffectsMode))

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