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/persistence/PersistenceQueryDocTest.java

@@ -451,15 +451,20 @@ public class PersistenceQueryDocTest {
 
     public TheOneWhoWritesToQueryJournal() {
       store = new ExampleStore();
-
-      receive(ReceiveBuilder.matchAny(message -> {
-        state = updateState(state, message);
-
-        // example saving logic that requires state to become ready:
-        if (state.readyToSave())
-          store.save(Record.of(state));
-        
-      }).build());
+    }
+    
+    @Override
+    public Receive createReceive() {
+      return receiveBuilder()
+        .matchAny(message -> {
+          state = updateState(state, message);
+  
+          // example saving logic that requires state to become ready:
+          if (state.readyToSave())
+            store.save(Record.of(state));
+          
+        })
+        .build();
     }