Add changelog note

Author: github-actions[bot]

CHANGELOG.md

@@ -6,7 +6,7 @@ Note that the only difference between `v2` and `v3` of the CodeQL Action is the
 
 ## [UNRELEASED]
 
-No user facing changes.
+- Update default CodeQL bundle version to 2.18.1. [#2385](https://github.com/github/codeql-action/pull/2385)
 
 ## 3.25.13 - 19 Jul 2024