docs: add changelog entry

Author: DannyvdSluijs

CHANGELOG.md

@@ -11,6 +11,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - Added extra breaking change to UPDATE-6.0.md regarding BaseConstraint::addError signature change ([#823](https://github.com/jsonrainbow/json-schema/pull/823)
+- Update constraint class to PHP 7.2 language level ([#824](https://github.com/jsonrainbow/json-schema/pull/824)
+
 
 ## [6.4.1] - 2025-04-04
 ### Fixed