remove 2.21 (mongodb#332) (mongodb#335)

(cherry picked from commit b3a1e69)

Author: github-actions[bot]

source/whats-new.txt

@@ -212,22 +212,4 @@ The 2.22 driver release includes the following new features:
 - Reduced memory allocation when using encryption.
 - Added logging messages for server discovery and monitoring (SDAM) events. For more
   information on these events, see the
-  `MongoDB SDAM Logging and Monitoring specification. <https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring-logging-and-monitoring.rst#log-messages>`__
-
-.. _version-2.21:
-
-What's New in 2.21
-------------------
-
-.. warning:: Breaking changes in v2.21
-
-   The v2.21 driver contains breaking changes. See :ref:`<csharp-breaking-changes-2.21.0>`
-   for more information.
-
-The 2.21 driver release includes the following new features:
-
-- Improvements and bug fixes to the LINQ3 integration.
-- Accepts options for Atlas Search sorting, tracking, and scoring.
-- Supports the Atlas Search :atlas:`embeddedDocument </atlas-search/embedded-document/>` operator.
-- Offers an API for Atlas Search index management.
-- Accepts anonymous types in the ``ObjectSerializer.DefaultAllowedTypes`` method.
+  `MongoDB SDAM Logging and Monitoring specification. <https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring-logging-and-monitoring.rst#log-messages>`__