Fix MissingTemplate error after Rails 8 upgrade

[akakiyo]

Before this update, SimpleJson expected to generate the template_name in options[:template]. However, with the Rails 8 upgrade (ref: rails/rails#52984), a value is no longer passed to options[:template].
For example:

Error: test: The renamed template correctly renders a JSON(ActionControllerAPITest):
  ActionView::MissingTemplate: Missing template api/posts/show with {:locale=>[:en], :formats=>[:json], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :jb, :jbuilder]}.

Therefore, I modified it to generate the template_name from options[:action].