introduce a StreamLimitReachedError for Connection.Open{Uni}Stream (#4579)

Using a concrete type is preferable to relying on interpreting the
return values from net.Error.Timeout and net.Error.Temporary, especially
since the latter has been deprecated since Go 1.18.

streams_map_outgoing.go

@@ -60,7 +60,7 @@ func (m *outgoingStreamsMap[T]) OpenStream() (T, error) {
 	// if there are OpenStreamSync calls waiting, return an error here
 	if len(m.openQueue) > 0 || m.nextStream > m.maxStream {
 		m.maybeSendBlockedFrame()
-		return *new(T), streamOpenErr{errTooManyOpenStreams}
+		return *new(T), streamOpenErr{&StreamLimitReachedError{}}
 	}
 	return m.openStream(), nil
 }