Merge pull request #19835 from agolubev/19818-Flow.transform-deprected-agolubev

=str #19818 Flow.transform should be deprected too

akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala

@@ -988,6 +988,7 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * This operator makes it possible to extend the `Flow` API when there is no specialized
    * operator that performs the transformation.
    */
+  @deprecated("Use via(GraphStage) instead.", "2.4.3")
   def transform[U](mkStage: function.Creator[Stage[Out, U]]): javadsl.Flow[In, U, Mat] =
     new Flow(delegate.transform(() ⇒ mkStage.create()))