Add ApplicationRecord as new superclass for app models

https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#active-record-models-now-inherit-from-applicationrecord-by-default

app/models/application_record.rb

@@ -0,0 +1,3 @@
+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end

app/models/article.rb

@@ -22,7 +22,7 @@
 #  category       :string(255)
 #
 
-class Article < ActiveRecord::Base
+class Article < ApplicationRecord
   belongs_to :supplier
   
   validates_numericality_of :price, :tax, :deposit, :unit_quantity

app/models/supplier.rb

@@ -1,4 +1,4 @@
-class Supplier < ActiveRecord::Base
+class Supplier < ApplicationRecord
   has_many :articles, :dependent => :destroy
   has_many :user_accesses, :dependent => :destroy
   has_many :users, :through => :user_accesses

app/models/user.rb

@@ -1,4 +1,4 @@
-class User < ActiveRecord::Base
+class User < ApplicationRecord
 
   has_many :user_accesses, :dependent => :destroy
   has_many :suppliers, :through => :user_accesses

app/models/user_access.rb

@@ -1,4 +1,4 @@
-class UserAccess < ActiveRecord::Base
+class UserAccess < ApplicationRecord
   belongs_to :user
   belongs_to :supplier
 end