[WIP] Add changenote for deprecation

Author: angelapwen

CHANGELOG.md

@@ -2,11 +2,9 @@
 
 See the [releases page](https://github.com/github/codeql-action/releases) for the relevant changes to the CodeQL CLI and language packs.
 
-Note that the only difference between `v2` and `v3` of the CodeQL Action is the node version they support, with `v3` running on node 20 while we continue to release `v2` to support running on node 16. For example `3.22.11` was the first `v3` release and is functionally identical to `2.22.11`. This approach ensures an easy way to track exactly which features are included in different versions, indicated by the minor and patch version numbers.
-
 ## [UNRELEASED]
 
-No user facing changes.
+- CodeQL Action v2 is now deprecated, and is no longer updated or supported. For better performance, improved security, and new features, upgrade to v3. For more information, see [this changelog post](TODO). [#1467](https://github.com/github/codeql-action/pull/2677)
 
 ## 3.28.0 - 20 Dec 2024