Update checkout action to version 4

We were getting some warnings with version 3:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/checkout@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

.github/workflows/linters.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - run: |
           git fetch --no-tags --prune origin +refs/heads/*:refs/remotes/origin/*
       - name: Setup Ruby