Turn off enableFormActions in Meta build (facebook#26721)

This is enabled in the canary channels, but because it's relatively
untested, we'll disable it at Meta until they're ready to start trying
it out. It can change some behavior even if you don't intentionally
start using the API.

The reason it's not a dynamic flag is that it affects the external Fizz
runtime, which currently can't read flags at runtime.

packages/shared/forks/ReactFeatureFlags.www.js

@@ -73,7 +73,7 @@ export const enableLegacyCache = true;
 export const enableCacheElement = true;
 export const enableFetchInstrumentation = false;
 
-export const enableFormActions = true;
+export const enableFormActions = false;
 
 export const disableJavaScriptURLs = true;