DOCSP-43732 - Remove [Serializable] (#237)

Author: mongoKart

source/upgrade.txt

@@ -76,6 +76,11 @@ Version 3.0 Potential Breaking Change
 - The LINQ2 provider has been removed from this version of the driver.
   You must use LINQ3 for all LINQ queries.
 
+- Exception classes and their related types no longer contain the ``[Serializable]``
+  attribute, and therefore no longer support the Microsoft legacy serialization API. To
+  learn how to use the {+driver-short+} to serialize objects, see the
+  :ref:`csharp-serialization` guide.
+
 - TLS 1.0 and 1.1 are no longer supported. You must use TLS 1.2 or higher. To learn
   more about configuring TLS/SSL in the {+driver-short+}, see :ref:`<csharp-tls>`.