Use [Unreleased] in CHANGELOG

Author: PokhodenkoSA

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Next Release] TBD
+## [Unreleased]
 
 ### Added
 - Extending `dpctl.device_context` with nested contexts (#678)