Support chages to show_helper in Rails 7

[rickselby]

This is a continuation of #2741

#2492 overrode show_helper, which in Rails 6 looks like:

def show_helper
  "#{singular_route_name}_url(@#{singular_table_name})"
end

But Rails 7 changed it to

def show_helper(arg = "@#{singular_table_name}", type: :url) # :doc:
  "#{singular_route_name}_#{type}(#{arg})"
end

This change supports the arguments in Rails 7 while maintaining support for Rails 6.

I've updated the edit_spec.rb template to use the show_helper helper rather than building the path itself, and have added a test to confirm the expected path is generated.