CI against Ruby 3.1

Author: amatsuda

.github/workflows/ruby.yml

@@ -17,6 +17,7 @@ jobs:
         ruby:
           - 2.7
           - "3.0"
+          - 3.1
         gemfile:
           - gemfiles/Gemfile-rails.5.2.x
           - gemfiles/Gemfile-rails.6.0.x
@@ -26,6 +27,8 @@ jobs:
             gemfile: gemfiles/Gemfile-rails.6.1.x
           - ruby: "3.0"
             gemfile: gemfiles/Gemfile-rails.5.2.x
+          - ruby: 3.1
+            gemfile: gemfiles/Gemfile-rails.5.2.x
         experimental: [false]
         include:
           - ruby: 2.7