CI against Ruby 3.4

Ruby 3.4 has been released, and support for Ruby 3.4.0 and 3.4.1 was added to setup-ruby:
ruby/setup-ruby#679

Author: koic

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu]
-        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', 'head']
+        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', 'head']
 
     steps:
       - name: checkout