[FB] disable legacy mode in modern build

Disables legacy mode in www-modern and removes the entrypoint hack in tests since we gate legacy mode tests now

Additionally disables test utils because this relies on legacy mode.

Additionally gates react-test-renderer test for unstable_Scope because it seems to also rely on legacy roots.

Author: gnoff

packages/react-dom/src/__tests__/ReactDOMRoot-test.js

@@ -47,7 +47,7 @@ describe('ReactDOMRoot', () => {
     expect(container.textContent).toEqual('Hi');
   });
 
-  // @gate !classic || !__DEV__
+  // @gate !disableLegacyMode
   it('warns if you import createRoot from react-dom', async () => {
     expect(() => ReactDOM.createRoot(container)).toErrorDev(
       'You are importing createRoot from "react-dom" which is not supported. ' +
@@ -58,7 +58,7 @@ describe('ReactDOMRoot', () => {
     );
   });
 
-  // @gate !classic || !__DEV__
+  // @gate !disableLegacyMode
   it('warns if you import hydrateRoot from react-dom', async () => {
     expect(() => ReactDOM.hydrateRoot(container, null)).toErrorDev(
       'You are importing hydrateRoot from "react-dom" which is not supported. ' +

scripts/jest/setupHostConfigs.js

@@ -14,11 +14,6 @@ function resolveEntryFork(resolvedEntry, isFBBundle) {
   // .js
 
   if (isFBBundle) {
-    if (__EXPERIMENTAL__) {
-      // We can't currently use the true modern entry point because too many tests fail.
-      // TODO: Fix tests to not use ReactDOM.render or gate them. Then we can remove this.
-      return resolvedEntry;
-    }
     const resolvedFBEntry = resolvedEntry.replace(
       '.js',
       __EXPERIMENTAL__ ? '.modern.fb.js' : '.classic.fb.js'