Update upgrade.txt

Author: rustagir

source/reference/upgrade.txt

@@ -435,6 +435,16 @@ end-of-life (EOL).
 To learn more about the MongoDB support for EOL products,
 see the `Legacy Support Policy <https://www.mongodb.com/support-policy/legacy>`__.
 
+.. _java-server-8.1-incompatibility:
+
+Server Version 8.1 Support Changes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You cannot use a 3.x version of the {+driver-short+} to connect to a
+deployment running {+mdb-server+} v8.1. Starting in {+mdb-server+} v8.1,
+the ``buildinfo`` command requires authentication, causing an
+incompatibility with the v3.x driver.
+
 .. _java-server-release-change-v4.8:
 
 Version 4.8 Server Release Support Changes