improve AbstractActor, #21717

* Receive class that wraps PartialFunction, to avoid
  scary scala types
* move AbstractActorContext to AbstractActor.ActorContext
* converting docs, many, many UntypedActor
* removing UntypedActor docs
* add unit test for ReceiveBuilder
* MiMa filters
* consistent use of getContext(), self(), sender()
* rename cross references
* migration guide
* skip samples for now
* improve match type safetyi, add matchUnchecked
  * the `? extends P` caused code like this to compile:
    `match(String.class, (Integer i) -> {})`
  * added matchUnchecked, since it can still be useful (um, convenient)
    to be able to do:
    `matchUnchecked(List.class, (List<String> list) -> {})`
* eleminate some scala.Option
  * preRestart
  * findChild
  * ActorIdentity.getActorRef

akka-docs/rst/java/code/docs/actor/SchedulerDocTest.java

@@ -11,10 +11,11 @@ import java.util.concurrent.TimeUnit;
 //#imports1
 
 //#imports2
-import akka.actor.UntypedActor;
 import akka.actor.Cancellable;
 //#imports2
 
+import docs.actorlambda.MyActor;
+import akka.actor.AbstractActor;
 import akka.actor.ActorRef;
 import akka.actor.ActorSystem;
 import akka.testkit.AkkaSpec;
@@ -28,7 +29,7 @@ public class SchedulerDocTest extends AbstractJavaTest {
       AkkaSpec.testConf());
 
   private final ActorSystem system = actorSystemResource.getSystem();
-  private ActorRef testActor = system.actorOf(Props.create(MyUntypedActor.class));
+  private ActorRef testActor = system.actorOf(Props.create(MyActor.class));
 
   @Test
   public void scheduleOneOffTask() {
@@ -51,14 +52,14 @@ public class SchedulerDocTest extends AbstractJavaTest {
   @Test
   public void scheduleRecurringTask() {
     //#schedule-recurring
-    class Ticker extends UntypedActor {
+    class Ticker extends AbstractActor {
       @Override
-      public void onReceive(Object message) {
-        if (message.equals("Tick")) {
-          // Do someting
-        } else {
-          unhandled(message);
-        }
+      public Receive createReceive() {
+        return receiveBuilder()
+          .matchEquals("Tick", m -> {
+            // Do someting
+          })
+          .build();
       }
     }