Use actions/checkout@v4 for Node 20

Author: fniephaus

.github/workflows/main.yml

@@ -80,7 +80,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     name: ${{ matrix.smalltalk }} on ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup smalltalkCI
         uses: hpi-swa/setup-smalltalkCI@v1
         with: