Update changelog for v2.21.0

Author: github-actions[bot]

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 See the [releases page](https://github.com/github/codeql-action/releases) for the relevant changes to the CodeQL CLI and language packs.
 
-## [UNRELEASED]
+## 2.21.0 - 19 Jul 2023
 
 - CodeQL Action now requires CodeQL CLI 2.9.4 or later. For more information, see the corresponding changelog entry for CodeQL Action version 2.20.4. [#1724](https://github.com/github/codeql-action/pull/1724)