Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/check-renovatebot-config.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Validate
         uses: suzuki-shunsuke/github-action-renovate-config-validator@v1.1.1

.github/workflows/test-import-export.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Build Docker image
         run: ${{ github.workspace }}/build_docker_image.sh