Bump actions/checkout from v2 to v4

Author: mishina2228

.github/workflows/main.yml

@@ -22,7 +22,7 @@ jobs:
 
  steps:
  - name: Checkout code
- uses: actions/checkout@v2
+ uses: actions/checkout@v4
 
  - name: Setup Ruby
  uses: ruby/setup-ruby@v1