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

Author: unity-renovate[bot]

.github/workflows/pr-description-validation.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Check PR description
         uses: actions/github-script@v8

.github/workflows/pr-supervisor.yaml

@@ -25,7 +25,7 @@ jobs:
     timeout-minutes: 720
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Wait and Verify Yamato Job Status
         env: