Update changelog for v2.21.1

Author: github-actions[bot]

Diff for: 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]
+## 2.21.1 - 26 Jul 2023
 
 - Improve the handling of fatal errors from the CodeQL CLI. [#1795](https://github.com/github/codeql-action/pull/1795)
 - Add the `sarif-output` output to the analyze action that contains the path to the directory of the generated SARIF. [#1799](https://github.com/github/codeql-action/pull/1799)