docs: prepare release 6.4.0

Author: DannyvdSluijs

CHANGELOG.md

@@ -6,6 +6,8 @@ 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]
+
+## [6.4.0] - 2025-04-01
 ### Added
 - Run PHPStan using the lowest and highest php version ([#811](https://github.com/jsonrainbow/json-schema/pull/811))
 ### Fixed