Update actions/checkout action to v4

.github/workflows/linting.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checks out repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Set up Python 3.9
         id: python

.github/workflows/push-container.yml

@@ -25,7 +25,7 @@ jobs:
 
       # Check out the current repository in the `cloudflare-ddns-docker` subdirectory
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           path: cloudflare-ddns-docker