Update actions/checkout to v4

Author: schinery

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
 
     steps:
       - name:                     "Checkout Code"
-        uses:                     "actions/checkout@v2"
+        uses:                     "actions/checkout@v4"
         timeout-minutes:          5
         with:
           fetch-depth:            0