Add changelog note

Author: henrymercer

CHANGELOG.md

@@ -4,7 +4,7 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 ## [UNRELEASED]
 
-No user facing changes.
+- The 'cleanup-level' input to the 'init' Action is now deprecated. The CodeQL Action has not written intermediate results to the database since version 2.2.5, so this option now has little to no practical use. [#2998](https://github.com/github/codeql-action/pull/2998)
 
 ## 3.29.5 - 29 Jul 2025