Update upgrade.txt

Author: rustagir

source/upgrade.txt

@@ -28,7 +28,9 @@ Overview
 --------
 
 Use the guides on this page to identify the potentially *breaking changes* that each
-version of the {+driver-short+} introduces. 
+version of the {+driver-short+} introduces. This page also provides information about
+breaking compatibility changes between the driver and MongoDB.
+
 A breaking change is a modification of a convention or a behavior starting in
 a specific version of the driver. This type of change may prevent your application from
 working properly if you don't address it.
@@ -45,4 +47,29 @@ Upgrade Guides
 --------------
 
 - :ref:`csharp-upgrade-v2`
-- :ref:`csharp-upgrade-v3`
+- :ref:`csharp-upgrade-v3`
+
+.. _csharp-server-release-changes:
+
+Server Release Compatibility Changes
+------------------------------------
+
+A server release compatibility change is a modification
+to the {+driver-long+} that discontinues support for a set of
+{+mdb-server+} versions.
+
+The driver discontinues support for a {+mdb-server+} version after it reaches
+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>`__.
+
+.. _csharp-server-8.1-incompatibility:
+
+Server Version 8.1 Support Changes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You cannot use v2.14.1 or earlier versions 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 these driver versions.