CI: Use actions/checkout@v4 to avoid deprecation warnings

Author: olleolleolle

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
         ruby-version: ["3.4", "3.3", "3.2", "3.1", "3.0", "2.7", "2.6", "2.5", "2.4", "2.3"]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby-version }}
         uses: ruby/setup-ruby@v1
         with: