chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/pr-auditor.yml

@@ -9,7 +9,7 @@ jobs:
   check-pr:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           repository: 'sourcegraph/devx-service'
           token: ${{ secrets.PR_AUDITOR_TOKEN }}