Update checkout in GitHub Actions

Author: sim642

.github/workflows/sbt.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up JDK 21
       uses: actions/setup-java@v2
       with: