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

Author: renovate[bot]

.github/workflows/tflint.yml

@@ -14,7 +14,7 @@ jobs:
         os: [ubuntu-latest]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         name: Checkout source code
 
       - uses: actions/cache@v4