Update GitHub checkout action from v3 to v4

.github/workflows/unittest.yaml

@@ -30,7 +30,7 @@ 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: Install dependencies
         run: |