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/testkit/TestKitDocTest.java

@@ -37,7 +37,7 @@ public class TestKitDocTest {
   static class MyActor extends UntypedActor {
     public void onReceive(Object o) throws Exception {
       if (o.equals("say42")) {
-        getSender().tell(42, getSelf());
+        sender().tell(42, self());
       } else if (o instanceof Exception) {
         throw (Exception) o;
       }