Remove deprecated calls to routing mapper

See rails/rails#52422

config/routes.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
 LetterOpenerWeb::Engine.routes.draw do
-  get  '/'                     => 'letters#index',    as: :letters
-  post 'clear'                 => 'letters#clear',    as: :clear_letters
-  get  ':id(/:style)'          => 'letters#show',     as: :letter
-  post ':id/delete'            => 'letters#destroy',  as: :delete_letter
-  get  ':id/attachments/:file' => 'letters#attachment', constraints: { file: %r{[^/]+} }
+  get  '/',                     to: 'letters#index',    as: :letters
+  post 'clear',                 to: 'letters#clear',    as: :clear_letters
+  get  ':id(/:style)',          to: 'letters#show',     as: :letter
+  post ':id/delete',            to: 'letters#destroy',  as: :delete_letter
+  get  ':id/attachments/:file', to: 'letters#attachment', constraints: { file: %r{[^/]+} }
 end

spec/dummy/config/routes.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 Rails.application.routes.draw do
-  mount LetterOpenerWeb::Engine => '/letter_opener_web'
+  mount LetterOpenerWeb::Engine, at: '/letter_opener_web'
 end