Throw when act is used in production (#21686)

Upgrades the deprecation warning to a runtime error.

I did it this way instead of removing the export so the type is the same
in both builds. It will get dead code eliminated regardless.

scripts/error-codes/codes.json

@@ -393,5 +393,6 @@
   "402": "The depth must equal at least at zero before reaching the root. This is a bug in React.",
   "403": "Tried to pop a Context at the root of the app. This is a bug in React.",
   "404": "Invalid hook call. Hooks can only be called inside of the body of a function component.",
-  "405": "hydrateRoot(...): Target container is not a DOM element."
+  "405": "hydrateRoot(...): Target container is not a DOM element.",
+  "406": "act(...) is not supported in production builds of React."
 }