Add changelog entry for v3.28.20 backport

Author: koesie10

CHANGELOG.md

@@ -20,6 +20,10 @@ No user facing changes.
 - Update default CodeQL bundle version to 2.22.0. [#2925](https://github.com/github/codeql-action/pull/2925)
 - Bump minimum CodeQL bundle version to 2.16.6. [#2912](https://github.com/github/codeql-action/pull/2912)
 
+## 3.28.20 - 21 July 2025
+
+- Remove support for combining SARIF files from a single upload for GHES 3.18, see [the changelog post](https://github.blog/changelog/2024-05-06-code-scanning-will-stop-combining-runs-from-a-single-upload/). [#2959](https://github.com/github/codeql-action/pull/2959)
+
 ## 3.28.19 - 03 Jun 2025
 
 - The CodeQL Action no longer includes its own copy of the extractor for the `actions` language, which is currently in public preview.