Set version 6.7 and release date in changelog

Merged-by: Martin Grzenia <martin.grzenia@iml.fraunhofer.de>

Author: swltr

opentcs-documentation/src/docs/release-notes/changelog.adoc

@@ -16,7 +16,7 @@ The openTCS developers
 This change log lists the most relevant changes for past releases in reverse chronological order.
 (Note that the openTCS project adheres to https://semver.org/[Semantic Versioning].)
 
-== Unreleased
+== Version 6.7 (2025-10-22)
 
 * New features and enhancements:
 ** Add new version 7.0.0 of the XML Schema definition for the openTCS plant model.