Bump actions/checkout

Author: m-nakamura145

.github/workflows/tests.yml

@@ -18,7 +18,7 @@ jobs:
           - truffleruby-head
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true