Codemod act -> await act (3/?) (#26336)

Similar to the rationale for `waitFor` (see #26285), we should always
await the result of an `act` call so that microtasks have a chance to
fire.

This only affects the internal `act` that we use in our repo, for now.
In the public `act` API, we don't yet require this; however, we
effectively will for any update that triggers suspense once `use` lands.
So we likely will start warning in an upcoming minor.