Upgrade actions/checkout to v4

Fix the following Github actions warning:

"The following actions use a deprecated Node.js version and will be
forced to run on node20: actions/checkout@v2. For more info:
https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/"

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
         emacs_version: [25, 26, 27, 28]
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: CI
         env:
           VERSION: ${{ matrix.emacs_version }}