Update actions/checkout action to v4 (#744)

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     name: Test code blocks in markdown
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update stable
 
@@ -31,7 +31,7 @@ jobs:
     name: Test tutorial-code directory
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update stable