Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/iso-processor.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v3