chore(deps): bump actions/checkout from 4 to 6 (#39)

Author: dependabot[bot]

.github/workflows/boj-build.yml

@@ -9,7 +9,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        uses: actions/checkout@v6
+
       - name: Trigger BoJ Server (Casket/ssg-mcp)
         run: |
           # Send a secure trigger to boj-server to build this repository