add note about backporting check changes to v2 branch

Author: nickfyson

CONTRIBUTING.md

@@ -76,7 +76,9 @@ Since the `codeql-action` runs most of its testing through individual Actions wo
 
 1. By default, this script retrieves the checks from the latest SHA on `main`, so make sure that your `main` branch is up to date.
 2. Run the script. If there's a reason to, you can pass in a different SHA as a CLI argument.
-3. After running, go to the [branch protection rules settings page](https://github.com/github/codeql-action/settings/branches) and validate that the rules for `main`, `v1`, and `v2` have been updated.
+3. After running, go to the [branch protection rules settings page](https://github.com/github/codeql-action/settings/branches) and validate that the rules for `main`, `v2`, and `v3` have been updated.
+
+Note that any updates to checks need to be backported to the `releases/v2` branch, in order to maintain the same set of names for required checks.
 
 ## Deprecating a CodeQL version (write access required)