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/stream/IntegrationDocTest.java

@@ -255,13 +255,18 @@ public class IntegrationDocTest extends AbstractJavaTest {
   static class DatabaseService extends AbstractActor {
     public final ActorRef probe;
 
-    DatabaseService(ActorRef probe) {
+    public DatabaseService(ActorRef probe) {
       this.probe = probe;
-
-      receive(ReceiveBuilder.match(Save.class, s -> {
-        probe.tell(s.tweet.author.handle, ActorRef.noSender());
-        sender().tell(SaveDone.INSTANCE, self());
-      }).build());
+    }
+    
+    @Override
+    public Receive createReceive() {
+      return receiveBuilder()
+        .match(Save.class, s -> {
+          probe.tell(s.tweet.author.handle, ActorRef.noSender());
+          sender().tell(SaveDone.INSTANCE, self());
+        })
+        .build();
     }
   }
 
@@ -298,7 +303,7 @@ public class IntegrationDocTest extends AbstractJavaTest {
         // ... process message
         String reply = word.toUpperCase();
         // reply to the ask
-        getSender().tell(reply, getSelf());
+        sender().tell(reply, self());
       } else {
         unhandled(message);
       }