Point Futures.asJava deprecation to CompletionStages.fromScala

Motivation:
Users should be directed to the new CompletionStages.fromScala method
rather than the low-level scala.jdk.FutureConverters.

Modification:
Update deprecation message on Futures.asJava.

Result:
Consistent migration path through CompletionStages API.

Tests:
- Not run - message-only change

References:
Fixes #1417

Author: He-Pin

actor/src/main/scala/org/apache/pekko/dispatch/Future.scala

@@ -90,7 +90,7 @@ object Futures {
    *
    * @since 1.2.0
    */
-  @deprecated("Use scala.jdk.FutureConverters instead.", since = "2.0.0")
+  @deprecated("Use CompletionStages.fromScala instead.", since = "2.0.0")
   def asJava[T](future: Future[T]): CompletionStage[T] = {
     import scala.jdk.FutureConverters._
     future.asJava