client/finality-granpda/until_imported: Rework pinning (#5983)

An `UntilImported` stream wraps a `Stream` of incoming messages and
waits for blocks those messages are based on before passing the messages
on.

The above `Stream` of incoming messages implements `Unpin`, thus there
is no need to use `pin_project` on the `UntilImported` struct. Instead
one only has to add the `Unpin` trait bound on the `I` trait parameter.