Update changelog for v2.20.0

Author: github-actions[bot]

CHANGELOG.md

@@ -1,6 +1,6 @@
 # CodeQL Action Changelog
 
-## [UNRELEASED]
+## 2.20.0 - 13 Jun 2023
 
 - Bump the version of the Action to 2.20.0. This ensures that users who received a Dependabot upgrade to [`cdcdbb5`](https://github.com/github/codeql-action/commit/cdcdbb579706841c47f7063dda365e292e5cad7a), which was mistakenly marked as Action version 2.13.4, continue to receive updates to the CodeQL Action. Full details in [#1729](https://github.com/github/codeql-action/pull/1729)