!str #16039 Remove old scaladsl, rename scaladsl2

* and impl2, testkit2
* keeping io2 for now

Are there any more things in impl2 that are not used?

One failing test (ignored for now): io2.TcpFlowSpec
See FIXME in that test. What can I have done?
the issue is that it never receives this `ActorRef`. it is not null as I first thought. I have no clue what is causing this. Any hints would be appreciated.