docs changelog: remove unreleased header from 2.1.0 docs

Author: jamiebrynes7

docs/versioned_docs/version-2.1.0/changelog.md

@@ -10,8 +10,6 @@ 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).
 
-## Unreleased
-
 ## v2.1.0 (2025-07-13)
 
 ### ✨ Features