gha: use checkout@v4 action instead of checkout@v2

.github/workflows/c-compile.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: build the NAT46 module
       run: |
         cd nat46/modules