doc: update CHANGELOG for v2.2.1 (#9861)

Author: coryan

CHANGELOG.md

@@ -116,6 +116,10 @@ may elicit a deprecation warning. See [#9086] for more details.
 
 </details>
 
+## v2.2.1 - 2022-09
+
+- Update Protobuf version in Bazel configuration to v21.6 (#9856)
+
 ## v2.2.0 - 2022-09
 
 ### New Libraries