ci: bump actions/checkout from 5 to 6 (#92)

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Author: dependabot[bot]

.github/workflows/build-deploy.yml

@@ -10,7 +10,7 @@ jobs:
     name: Build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v2