Sets release notes for 1.21.0

Author: tdstein

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).
 
-## Unreleased
+## [1.21.0] - 2023-10-26
 
 ### Fixed
 - The `https_proxy` environment variable is recognized as a synonym for