Rename "shared subset" to "server" (#27939)

The internal file ReactSharedSubset is what the `react` module resolves
to when imported from a Server Component environment. We gave it this
name because, originally, the idea was that Server Components can access
a subset of the APIs available on the client.

However, since then, we've also added APIs that can _only_ by accessed
on the server and not the client. In other words, it's no longer a
subset, it's a slightly different overlapping set.

So this commit renames ReactSharedSubet to ReactServer and updates all
the references. This does not affect the public API, only our internal
implementation.