Merge pull request #20325 from akka/wip-convert-scan-to-graphstage-√

Converts the Scan-operation from PushPullStage to GraphStage

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

@@ -768,7 +768,7 @@ trait FlowOps[+Out, +Mat] {
    *
    * '''Cancels when''' downstream cancels
    */
-  def scan[T](zero: T)(f: (T, Out) ⇒ T): Repr[T] = andThen(Scan(zero, f))
+  def scan[T](zero: T)(f: (T, Out) ⇒ T): Repr[T] = via(Scan(zero, f))
 
   /**
    * Similar to `scan` but only emits its result when the upstream completes,