Add ActiveModel.deprecator

This commit adds `ActiveModel.deprecator`, and adds it to
`Rails.application.deprecators` so that it can be configured via
settings such as `config.active_support.report_deprecations`.

activemodel/lib/active_model.rb

@@ -26,6 +26,7 @@
 require "active_support"
 require "active_support/rails"
 require "active_model/version"
+require "active_model/deprecator"
 
 module ActiveModel
   extend ActiveSupport::Autoload

activemodel/lib/active_model/deprecator.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module ActiveModel
+  def self.deprecator # :nodoc:
+    @deprecator ||= ActiveSupport::Deprecation.new
+  end
+end

activemodel/lib/active_model/railtie.rb

@@ -9,6 +9,10 @@ module ActiveModel
 
     config.active_model = ActiveSupport::OrderedOptions.new
 
+    initializer "active_model.deprecator" do |app|
+      app.deprecators[:active_model] = ActiveModel.deprecator
+    end
+
     initializer "active_model.secure_password" do
       ActiveModel::SecurePassword.min_cost = Rails.env.test?
     end

activemodel/test/cases/helper.rb

@@ -4,7 +4,7 @@ require "active_support/testing/strict_warnings"
 require "active_model"
 
 # Show backtraces for deprecated behavior for quicker cleanup.
-ActiveSupport::Deprecation.debug = true
+ActiveModel.deprecator.debug = true
 
 # Disable available locale checks to avoid warnings running the test suite.
 I18n.enforce_available_locales = false

railties/test/application/configuration_test.rb

@@ -3898,6 +3898,7 @@ module ApplicationTests
       assert_equal ActionText.deprecator, Rails.application.deprecators[:action_text]
       assert_equal ActionView.deprecator, Rails.application.deprecators[:action_view]
       assert_equal ActiveJob.deprecator, Rails.application.deprecators[:active_job]
+      assert_equal ActiveModel.deprecator, Rails.application.deprecators[:active_model]
       assert_equal ActiveRecord.deprecator, Rails.application.deprecators[:active_record]
       assert_equal ActiveStorage.deprecator, Rails.application.deprecators[:active_storage]
       assert_equal ActiveSupport.deprecator, Rails.application.deprecators[:active_support]