Switch ci.yaml to actions/checkout@v6

Author: eddelbuettel

.github/workflows/ci.yaml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup
         uses: eddelbuettel/github-actions/r-ci@master