Remove TLS 1.0 and 1.1 (mongodb#236)

Author: mongoKart

source/fundamentals/connection/tls.txt

@@ -20,6 +20,10 @@ configure your connection to use TLS/SSL, enable the TLS/SSL settings in either
 the :ref:`connection string <csharp-connection-uri>` or 
 :ref:`MongoClientSettings <csharp-mongo-client-settings>`.
 
+.. important:: TLS 1.2
+
+   The {+driver-short+} supports only TLS 1.2 or higher.
+
 .. _csharp-tls-enable:
 
 Enable TLS

source/upgrade.txt

@@ -72,6 +72,9 @@ 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.
 
+- 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>`.
+
 .. _csharp-breaking-changes-2.28.0:
 
 Version 2.28.0 Potential Breaking Change