Update to actions/checkout@v4.

Author: postmodern

.github/workflows/ruby.yml

@@ -17,7 +17,7 @@ jobs:
           - truffleruby
     name: Ruby ${{ matrix.ruby }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: