add ruby 4.0 to CI matrix

Author: kzkn

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby_version: ['3.4', '3.3', '3.2', '3.1', '3.0']
+        ruby_version: ['4.0', '3.4', '3.3', '3.2', '3.1', '3.0']
 
     steps:
     - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1