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/SubSource.scala

@@ -835,6 +835,7 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
    * 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]]): SubSource[U, Mat] =
     new SubSource(delegate.transform(() ⇒ mkStage.create()))