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

Author: renovate[bot]

.github/workflows/spec-validate.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Run OpenAPI spec validation
         uses: stoplightio/spectral-action@v0.8.13