Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/validate-openapi-spec.yml

@@ -11,7 +11,7 @@ jobs:
   format:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
       - uses: actions/setup-node@v6
         with:
           node-version: '24'