`when_all(just())` shouldn’t have a `set_stopped_t()` completion signature

If none of the senders it wraps are stoppable and the receiver it connects to doesn’t have a stop token, then there’s no way it could possibly stop!

as QoI it also shouldn’t have a stop source or stop token in the case of only-happy-path senders and unstoppable receiver (or if it does, they should be unstoppable).