Update actions/checkout to v6

GitHub deprecated Node.js 20 in Actions runners; the previous v4 release
runs on Node.js 20 and will be forced to Node.js 24 in June 2026. v6
runs on Node.js 24 natively.

Author: nertzy

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
             allow-failure: true
     continue-on-error: ${{ matrix.allow-failure }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: