Update actions to versions running on Node20

As per
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/,
GitHub has started a deprecation process for the GitHub Actions that run on
Node16. We're updating Actions that use this version of Node to newer versions,
running on Node20.

.github/workflows/code.yml

@@ -11,7 +11,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: actions/setup-node@v4
         with:
@@ -26,7 +26,7 @@ jobs:
   format:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: actions/setup-node@v4
         with:

.github/workflows/publish.yml

@@ -14,7 +14,7 @@ jobs:
     name: Build, test and publish
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           token: ${{ secrets.CI_GITHUB_TOKEN }}