fix(ci): update actions/checkout to v6 for Node.js 24 compatibility

Author: UtkarshUsername

.github/workflows/sync-upstream.yml

@@ -14,7 +14,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Fetch upstream OpenAPI spec
         run: |