Internal dispatcher to protect against starvation (#26816)

* Allow for dispatcher aliases and define a internal dispatcher
* Test checking dispatcher name
* MiMa for Dispatchers
* Migration guide entry
* No need to have custom dispatcher lookup logic in streams anymore
* Default dispatcher size and migration note about that
* Test checking exact config values...
* Typed receptionist on internal dispatcher
* All internal usages of system.dispatcher gone through

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

@@ -159,7 +159,9 @@ private[akka] final class FileSource(path: Path, chunkSize: Int, startPosition:
     val pub = try {
       val is = createInputStream() // can throw, i.e. FileNotFound
 
-      val props = InputStreamPublisher.props(is, ioResultPromise, chunkSize).withDispatcher(Dispatcher.resolve(context))
+      val props = InputStreamPublisher
+        .props(is, ioResultPromise, chunkSize)
+        .withDispatcher(context.effectiveAttributes.mandatoryAttribute[Dispatcher].dispatcher)
 
       val ref = materializer.actorOf(context, props)
       akka.stream.actor.ActorPublisher[ByteString](ref)