Release notes for 2.22.0.

Author: rstam

Release Notes/Release Notes v2.22.0.md

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+# .NET Driver Version 2.22.0 Release Notes
+
+This is the general availability release for the 2.22.0 version of the driver.
+
+The main new features in 2.22.0 include:
+
+* LINQ3 bug fixes and improvements
+* Support for $set aggregation stage
+* Added synonym support for Atlas Search
+* Support for $vectorSearch aggregation stage
+* Reduced memory allocations when using encryption
+* Additional logging messages
+
+An online version of these release notes is available [here](https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.22.0.md).
+
+The full list of issues resolved in this release is available at [CSHARP JIRA project](https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%202.22.0%20ORDER%20BY%20key%20ASC).
+
+Documentation on the .NET driver can be found [here](https://www.mongodb.com/docs/drivers/csharp/v2.22/).