CI: Upgrade actions/checkout to v4

Author: kleisauke

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
           - os: ubuntu-22.04
             nginx-version: 1.23.4
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install dependencies
         run: |
           # for Test::Nginx