Update CHANGELOG for 4.0.0 (#1279)

Author: eramongodb

CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 Changes prior to 3.9.0 are documented as [release notes on GitHub](https://github.com/mongodb/mongo-cxx-driver/releases).
 
-## 4.0.0 [Unreleased]
+## 4.0.0
 
 ## Added