Update CHANGELOG for heroku-20 EOL changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ipmb

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+* Warn when an EOL stack (`heroku-18` or `heroku-20`) is specified in `app.json`
+* Removed `heroku-20` integration test from CI (stack is end-of-life)
+
 ## [2.5.0] - 2025-10-21
 
 ### Added