Update upgrade.txt

(cherry picked from commit b73ee1a)

Author: rustagir

source/upgrade.txt

@@ -153,15 +153,14 @@ Version 2.13.0 Breaking Changes
 - The ``slave0k`` connection string option was removed; the
   ``readPreference`` option is now used instead.
 
-
 .. _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
-MongoDB Server versions.
+{+mdb-server+} versions.
 
 The driver discontinues support for a {+mdb-server+} version after it
 reaches end-of-life (EOL).
@@ -173,6 +172,16 @@ To learn more about the compatibility between .NET/C# driver versions
 and {+mdb-server+} versions, visit the
 :ref:`Compatibility <csharp-compatibility-tables>` page.
 
+.. _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.
+
 Version 2.14 Release Support Changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -181,4 +190,4 @@ use any driver from v2.14 and later, your {+mdb-server+} must be v3.6 or
 later.
 
 To learn how to upgrade your {+mdb-server+} deployment, see
-:manual:`Release Notes </release-notes/>` in the {+mdb-server+} manual.
+:manual:`Release Notes </release-notes/>` in the {+mdb-server+} manual.