Replace generic radio button with GitLab UI styled radio button in app/views/projects/_merge_request_merge_method_settings.html.haml

The HAML template app/views/projects/_merge_request_merge_method_settings.html.haml contains a reference to the f.radio_button form helper method which generates a vanilla <input type="radio" /> element. We want to migrate these to the styled radio buttons defined in the Pajamas design system.

There are rails helpers for radios and checkboxes which will generate these styled elements for you. You can read their documentation here.

Migration notes will appear in the associated epic (&7944)