Add note about last planned v2 release

Author: aeisenberg

CHANGELOG.md

@@ -4,6 +4,8 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 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.
 
+**This is the last planned release of the `v2`. To continue getting updates for the CodeQL Action, please switch to `v3`.**
+
 ## 2.28.0 - 20 Dec 2024
 
 - Bump the minimum CodeQL bundle version to 2.15.5. [#2655](https://github.com/github/codeql-action/pull/2655)