CXX-3193 add changelog entry dropping macOS 11 and macOS 12 (#1440)

Author: eramongodb

CHANGELOG.md

@@ -50,6 +50,7 @@ Changes prior to 3.9.0 are documented as [release notes on GitHub](https://githu
 - Support for MongoDB Server 4.2.
   - See: [MongoDB Software Lifecycle Schedules](https://www.mongodb.com/legal/support-policy/lifecycles).
   - See: [MongoDB C Driver 2.1.0 Release Notes](https://github.com/mongodb/mongo-c-driver/releases/tag/2.1.0).
+- Support for macOS 11 and macOS 12 (deprecated in 4.1.0).
 
 ## 4.1.1