Add CHANGELOG entry for CoreCLR API migration

- Added Unreleased section with fix note for Mono to CoreCLR API migration

Author: MiroBrodlovaUnityGit

com.unity.postprocessing/CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this package will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+- Replaced deprecated Mono API with CoreCLR-compatible API
+
 ## [3.5.0] - 2025-06-18
 
 ### Added