remove 2.21 (#332)

(cherry picked from commit b3a1e69)

Author: mongoKart

source/whats-new.txt

@@ -89,49 +89,3 @@ The 2.22 driver release includes the following new features:
 - 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.
-
-.. _version-2.20:
-
-What's New in 2.20
-------------------
-
-The 2.20 driver release includes the following new features:
-
-- Added full support for MongoDB Server version 7.0.0.
-- Added support for equality query types in Queryable Encryption GA.
-- Improvements to LINQ3 integration.
-- Improvements to serialization features.
-- Improvements to the logging framework.
-
-.. _version-2.19:
-
-What's New in 2.19
-------------------
-
-The 2.19 driver release includes the following new features:
-
-- The ``ObjectSerializer`` allows deserialization of only safe types, as determined by
-  the configurable ``AllowedTypes`` function.
-- Changed the default ``LinqProvider`` to LINQ3.
-- Added Atlas Search (``$search``) support to builders, aggregation pipelines, and LINQ3.
-- Added support for ``$bucket`` and ``$bucketAuto`` aggregation pipeline stages in LINQ3.
-- Added support for automatic KMS credentials for Azure VM Managed Identities.
-- Added native support for AWS IAM roles.