deduplicate logic for IODispatcher #24604 (#24619)

* deduplicate logic for IODispatcher #24604
 * introduce a resolveDispatcher helper in ActorAttributes
 * mention akka.stream.materializer.blocking-io-dispatcher instead of akka.stream.blocking-io-dispatcher in scaladocs
 * fix a flaky test
 * cosmetic changes in the touched files

* move resolveDispather helper to the Dispatcher companion object under a new name resolve

* filter out mima warning

* fix mima excludes after the 2.5.11 release

* address review comments
 * update stream-io.md with the correct dispatcher config key
 * mark ActorAttributes.Dispatcher#resolve as internal API
 * use the dispatche config key in ActorMaterializer

* add private[akka] to the resolve methods

akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala

@@ -12,7 +12,6 @@ import akka.Done
 import akka.annotation.InternalApi
 import akka.stream.ActorAttributes.Dispatcher
 import akka.stream.Attributes.InputBuffer
-import akka.stream.impl.Stages.DefaultAttributes.IODispatcher
 import akka.stream.impl.{ ErrorPublisher, SourceModule }
 import akka.stream.stage._
 import akka.stream.{ IOResult, _ }
@@ -155,12 +154,9 @@ private[akka] final class FileSource(path: Path, chunkSize: Int, startPosition:
     val pub = try {
       val is = createInputStream() // can throw, i.e. FileNotFound
 
-      val dispatcher = context.effectiveAttributes.mandatoryAttribute[Dispatcher] match {
-        case IODispatcher     ⇒ ActorMaterializerHelper.downcast(context.materializer).settings.blockingIoDispatcher
-        case Dispatcher(name) ⇒ name
-      }
-
-      val props = InputStreamPublisher.props(is, ioResultPromise, chunkSize).withDispatcher(dispatcher)
+      val props = InputStreamPublisher
+        .props(is, ioResultPromise, chunkSize)
+        .withDispatcher(Dispatcher.resolve(context))
 
       val ref = materializer.actorOf(context, props)
       akka.stream.actor.ActorPublisher[ByteString](ref)