Warn if you pass a hidden prop to Activity (#32916)

Since `hidden` is a prop on arbitrary DOM elements it's a common mistake
to think that it would also work that way on `<Activity>` but it
doesn't. In fact, we even had this mistakes in our own tests.

Maybe there's an argument that we should actually just support it but we
also have more modes planned.

So this adds a warning. It should also already be covered by TypeScript.