use v4 actions

.github/workflows/main.yml

@@ -32,7 +32,7 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Prepare CI
         run: |
           ln -s $GITHUB_WORKSPACE ~/flexbe_ci