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/extension/ExtensionDocTest.java

@@ -57,11 +57,16 @@ public class ExtensionDocTest extends AbstractJavaTest {
 
   static
   //#extension-usage-actor
-  public class MyActor extends UntypedActor {
-    public void onReceive(Object msg) {
-      // typically you would use static import of the
-      // CountExtension.CountExtensionProvider field
-      CountExtension.CountExtensionProvider.get(getContext().system()).increment();
+  public class MyActor extends AbstractActor {
+    @Override
+    public Receive createReceive() {
+      return receiveBuilder()
+        .matchAny(msg -> {
+          // typically you would use static import of the
+          // CountExtension.CountExtensionProvider field
+          CountExtension.CountExtensionProvider.get(getContext().system()).increment();
+        })
+        .build();
     }
   }