Bumped checkout to v4.

Author: Naenyn

.github/workflows/CI.yml

@@ -52,7 +52,7 @@ jobs:
     timeout-minutes: 5
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Set up JDK
         uses: actions/setup-java@v4
         with: