Update changelog for v2.1.21

Author: github-actions[bot]

CHANGELOG.md

@@ -1,6 +1,6 @@
 # CodeQL Action Changelog
 
-## [UNRELEASED]
+## 2.1.21 - 25 Aug 2022
 
 - Improve error messages when the code scanning configuration file includes an invalid `queries` block or an invalid `query-filters` block. [#1208](https://github.com/github/codeql-action/pull/1208)
 - Fix a bug where Go build tracing could fail on Windows. [#1209](https://github.com/github/codeql-action/pull/1209)