Update checkout action to v4

Author: eugeneius

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
           - 3.4
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby-version }}