Update changelog for v2.21.4

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]
+## 2.21.4 - 14 Aug 2023
 
 - Log a warning if the amount of available disk space runs low during a code scanning run. [#1825](https://github.com/github/codeql-action/pull/1825)
 - When downloading CodeQL bundle version 2.13.4 and later, cache these bundles in the Actions tool cache using a simpler version number. [#1832](https://github.com/github/codeql-action/pull/1832)