Update changelog for v2.21.3

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]
+## 2.21.3 - 08 Aug 2023
 
 - We are rolling out a feature in August 2023 that will improve multi-threaded performance on larger runners. [#1817](https://github.com/github/codeql-action/pull/1817)
 - Reduce disk space usage when downloading the CodeQL bundle. [#1820](https://github.com/github/codeql-action/pull/1820)