feat(after): allow using unstable_after in generateStaticParams (#73217)
Allows using `unstable_after` in `generateStaticParams`. definitely
niche, but since we're now allowing `after` to be used during build, i
see no reason not to.
Also fixes an issue where we didn't pass `experimental.after` into the
context used for `generateStaticParams`, so if `after` was called, it'd
always say that you have to enable `experimental.after` even if you
already did.