Update changelog for v2.20.1

Author: github-actions[bot]

Diff for: CHANGELOG.md

@@ -1,6 +1,6 @@
 # CodeQL Action Changelog
 
-## [UNRELEASED]
+## 2.20.1 - 21 Jun 2023
 
 - Update default CodeQL bundle version to 2.13.4. [#1721](https://github.com/github/codeql-action/pull/1721)
 - Experimental: add a new `resolve-environment` action which attempts to infer a configuration for the build environment that is required to build a given project. Do not use this in production as it is part of an internal experiment and subject to change at any time.