Update changelog

Author: richardwang1124

gems/aws-sdk-core/CHANGELOG.md

@@ -1,7 +1,7 @@
 Unreleased Changes
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-* Issue - Fix protocol selection for stubbed clients.
+* Issue - Prioritizes JSON over CBOR when both are supported for stubbed clients.
 
 3.239.2 (2025-11-25)
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