Add ruby 3 rails 5.2 to ignore matrix

Author: luke-hill

.github/workflows/build.yml

@@ -33,6 +33,7 @@ jobs:
           - { ruby: '2.7', gemfile: 'rails_5_1' }
           - { ruby: '3.0', gemfile: 'rails_5_0' }
           - { ruby: '3.0', gemfile: 'rails_5_1' }
+          - { ruby: '3.0', gemfile: 'rails_5_2' } # Rails 5.2 won't work with Ruby 3+: https://github.com/rails/rails/issues/40938
           - { ruby: '3.0', gemfile: 'rails_6_0' }
           - { ruby: '3.1', gemfile: 'rails_5_0' }
           - { ruby: '3.1', gemfile: 'rails_5_1' }