Update whats-new.txt

port changes from v4.10 what's new updates to v4.9

Author: rustagir

source/whats-new.txt

@@ -44,6 +44,14 @@ Upcoming Breaking Changes
 What's New in 4.9
 -----------------
 
+.. important:: Aggregation Expression Operations Are in Beta
+   
+   This driver version introduces an API for creating
+   :ref:`aggregation expressions <java-aggregation-expression-operations>`. 
+   This feature is in beta phase.
+
+New features of the 4.9 driver release include:
+
 - Added a new exception to identify when a constructor annotated by
   ``BsonCreator`` contains parameters with annotations other than
   ``BsonProperty`` or ``BsonId``.