Add changelog note

Author: mbg

CHANGELOG.md

@@ -4,7 +4,7 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 ## [UNRELEASED]
 
-No user facing changes.
+- Update default CodeQL bundle version to 2.23.4. [#3276](https://github.com/github/codeql-action/pull/3276)
 
 ## 4.31.2 - 30 Oct 2025