CI against Ruby 3.1

Author: amatsuda

.github/workflows/main.yml

@@ -18,11 +18,15 @@ jobs:
             gemfile: gemfiles/Gemfile-rails.7.0.x
             allow_failures: 'true'
 
+          - ruby_version: '3.1'
+            gemfile: gemfiles/Gemfile-rails.7.0.x
           - ruby_version: '3.0'
             gemfile: gemfiles/Gemfile-rails.7.0.x
           - ruby_version: '2.7'
             gemfile: gemfiles/Gemfile-rails.7.0.x
 
+          - ruby_version: '3.1'
+            gemfile: gemfiles/Gemfile-rails.6.1.x
           - ruby_version: '3.0'
             gemfile: gemfiles/Gemfile-rails.6.1.x
           - ruby_version: '2.7'