Update checkout action to latest version

Author: xuanxu

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         - ruby-version: '4.0'
           failure-allowed: true
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: