Update actions/checkout action to v6 (#9)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/validate.yml

@@ -11,5 +11,5 @@ jobs:
     name: validate
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: rinchsan/renovate-config-validator@v0.2.0