Prep changelog for v0.12.1 (#813)

Author: itsomri

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+## [v0.12.1] - 2025-12-25
+
 ### Added
 - Added the option to disable prometheus service monitor creation [#810](https://github.com/NVIDIA/KAI-Scheduler/pull/810) [itsomri](https://github.com/itsomri)