-str #16393 removes timerTransform from public API

removal of `timerTransform` from public APIs.
Removed completely from javadsl, hidden in scaladsl - because of use case in `toStrict`.

refs https://github.com/akka/akka/issues/16393
@ktoso yeah, I'm late to the party, but there are a few references to `timerTransform` in docs,
e.g. [5 lines above](https://github.com/akka/akka/pull/16397/files#diff-a4d5987f74d69093ccf1264366e4bbc1R288)