ci: bump actions/checkout to v6

Author: hellerve

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Carp
         uses: carpentry-org/setup-carp@v1