chore: Deprecate Source#future in javadsl

Author: He-Pin

stream-tests/src/test/java/org/apache/pekko/stream/javadsl/LazyAndFutureSourcesTest.java

@@ -43,6 +43,7 @@ public LazyAndFutureSourcesTest() {
   // note these are minimal happy path tests to cover API, more thorough tests are on the Scala side
 
   @Test
+  @SuppressWarnings("deprecation")
   public void future() throws Exception {
     CompletionStage<List<String>> result =
         Source.future(Future.successful("one")).runWith(Sink.seq(), system);

stream/src/main/scala/org/apache/pekko/stream/javadsl/Source.scala

@@ -391,6 +391,7 @@ object Source {
    *
    * Here for Java interoperability, the normal use from Java should be [[Source.completionStage]]
    */
+  @deprecated("Use 'Source.completionStage' instead", "Pekko 1.4.0")
   def future[T](futureElement: Future[T]): Source[T, NotUsed] =
     scaladsl.Source.future(futureElement).asJava