Update latest checkout action version

Author: m-nakamura145

Diff for: .github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         ruby: ["2.7", "3.0", "3.1", "3.2", "3.3", "ruby-head", "truffleruby-head", "jruby-9.4", "jruby-head"]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: rm Gemfile.lock
       - uses: ruby/setup-ruby@v1
         with: