ci: Add Ruby 3.4 x Rails 8.0

Author: toshimaru

.github/workflows/ci.yml

@@ -57,6 +57,8 @@ jobs:
             build-rails-versions: "5.2,6.0,6.1,7.0"
           - ruby-version: "3.3"
             build-rails-versions: "7.1,7.2"
+          - ruby-version: "3.4"
+            build-rails-versions: "8.0"
     steps:
       - uses: actions/checkout@v4
         with: