ci: Bump actions/checkout from 3 to 4

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install Stable Rust
         uses: actions-rust-lang/setup-rust-toolchain@v1