Upgrade checkout action

Author: vimpostor

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Build the Docker image
       run: docker build -t vim-lumen -f tests/Dockerfile .
     - name: Run tests with vim