Add changelog entry

CHANGELOG.md

@@ -2,7 +2,12 @@
 
 ## [UNRELEASED]
 
-No user facing changes.
+- 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. [#1729](https://github.com/github/codeql-action/pull/1729)
+  - The CodeQL Action repository contains a series of tags `v*` corresponding to versions of the CodeQL Action, as is typical. However it also contains a series of tags `codeql-bundle-*` that correspond to versions of the CodeQL Bundle, an artifact that contains the CodeQL CLI and the standard CodeQL libraries.
+  - As of version 2.13.4 of the CodeQL CLI, we changed the format of the CodeQL Bundle tag from a date, for example `codeql-bundle-20230613`, to a semantic version, for example `codeql-bundle-v2.13.4`.
+  - This inadvertently sent out Dependabot upgrades that upgraded users from `v2.3.6` to `codeql-bundle-v2.13.4`.
+  - To ensure that users who merged this Dependabot upgrade continue to receive updates to the CodeQL Action, we are bumping the Action version to make it greater than 2.13.4.
+  - To help avoid confusion between the version numbers of the CodeQL Action and the CodeQL CLI, we are introducing some separation and bumping the Action to 2.20.0.
 
 ## 2.3.6 - 01 Jun 2023