Update README to use actions/checkout@v4

Author: evansims

README.md

@@ -63,7 +63,7 @@ jobs:
     name: "Validation"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: gradle/wrapper-validation-action@v1
 ```