ci: add ruby-4.0

Author: yoshoku

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     name: Ruby ${{ matrix.ruby }}
     strategy:
       matrix:
-        ruby: [ '3.2', '3.3', '3.4' ]
+        ruby: [ '3.2', '3.3', '3.4', '4.0' ]
     steps:
       - uses: actions/checkout@v6
       - name: Set up Ruby