Fix primary_abstract_class with engines

With engines this would result in an error because there can only be one
`primary_abstract_class`. This is the class that your app connects to on
boot when it's connecting lazily and you're using multiple databases.
This change just removes `primary_abstract_class` from the engine
`ApplicationRecord` classes and keeps them as `self.abstract_class =
true`.

Fixes rails#43247

railties/lib/rails/generators/rails/plugin/templates/app/models/%namespaced_name%/application_record.rb.tt

@@ -1,6 +1,6 @@
 <%= wrap_in_modules <<~rb
   class ApplicationRecord < ActiveRecord::Base
-    primary_abstract_class
+    self.abstract_class = true
   end
 rb
 %>

railties/test/generators/plugin_generator_test.rb

@@ -425,7 +425,7 @@ def test_create_mountable_application_with_mountable_option_and_hypenated_name
     assert_file "hyphenated-name/lib/hyphenated/name.rb",                                    /require "hyphenated\/name\/engine"/
     assert_file "hyphenated-name/test/dummy/config/routes.rb",                               /mount Hyphenated::Name::Engine => "\/hyphenated-name"/
     assert_file "hyphenated-name/app/controllers/hyphenated/name/application_controller.rb", /module Hyphenated\n  module Name\n    class ApplicationController < ActionController::Base\n    end\n  end\nend\n/
-    assert_file "hyphenated-name/app/models/hyphenated/name/application_record.rb",          /module Hyphenated\n  module Name\n    class ApplicationRecord < ActiveRecord::Base\n      primary_abstract_class\n    end\n  end\nend/
+    assert_file "hyphenated-name/app/models/hyphenated/name/application_record.rb",          /module Hyphenated\n  module Name\n    class ApplicationRecord < ActiveRecord::Base\n      self.abstract_class = true\n    end\n  end\nend/
     assert_file "hyphenated-name/app/jobs/hyphenated/name/application_job.rb",               /module Hyphenated\n  module Name\n    class ApplicationJob < ActiveJob::Base/
     assert_file "hyphenated-name/app/mailers/hyphenated/name/application_mailer.rb",         /module Hyphenated\n  module Name\n    class ApplicationMailer < ActionMailer::Base\n      default from: "from@example\.com"\n      layout "mailer"\n    end\n  end\nend/
     assert_file "hyphenated-name/app/helpers/hyphenated/name/application_helper.rb",         /module Hyphenated\n  module Name\n    module ApplicationHelper\n    end\n  end\nend/
@@ -447,7 +447,7 @@ def test_create_mountable_application_with_mountable_option_and_hypenated_and_un
     assert_file "my_hyphenated-name/lib/my_hyphenated/name.rb",                                    /require "my_hyphenated\/name\/engine"/
     assert_file "my_hyphenated-name/test/dummy/config/routes.rb",                                  /mount MyHyphenated::Name::Engine => "\/my_hyphenated-name"/
     assert_file "my_hyphenated-name/app/controllers/my_hyphenated/name/application_controller.rb", /module MyHyphenated\n  module Name\n    class ApplicationController < ActionController::Base\n    end\n  end\nend\n/
-    assert_file "my_hyphenated-name/app/models/my_hyphenated/name/application_record.rb",          /module MyHyphenated\n  module Name\n    class ApplicationRecord < ActiveRecord::Base\n      primary_abstract_class\n    end\n  end\nend/
+    assert_file "my_hyphenated-name/app/models/my_hyphenated/name/application_record.rb",          /module MyHyphenated\n  module Name\n    class ApplicationRecord < ActiveRecord::Base\n      self.abstract_class = true\n    end\n  end\nend/
     assert_file "my_hyphenated-name/app/jobs/my_hyphenated/name/application_job.rb",               /module MyHyphenated\n  module Name\n    class ApplicationJob < ActiveJob::Base/
     assert_file "my_hyphenated-name/app/mailers/my_hyphenated/name/application_mailer.rb",         /module MyHyphenated\n  module Name\n    class ApplicationMailer < ActionMailer::Base\n      default from: "from@example\.com"\n      layout "mailer"\n    end\n  end\nend/
     assert_file "my_hyphenated-name/app/helpers/my_hyphenated/name/application_helper.rb",         /module MyHyphenated\n  module Name\n    module ApplicationHelper\n    end\n  end\nend/
@@ -469,7 +469,7 @@ def test_create_mountable_application_with_mountable_option_and_multiple_hypenat
     assert_file "deep-hyphenated-name/lib/deep/hyphenated/name.rb",                                    /require "deep\/hyphenated\/name\/engine"/
     assert_file "deep-hyphenated-name/test/dummy/config/routes.rb",                                    /mount Deep::Hyphenated::Name::Engine => "\/deep-hyphenated-name"/
     assert_file "deep-hyphenated-name/app/controllers/deep/hyphenated/name/application_controller.rb", /module Deep\n  module Hyphenated\n    module Name\n      class ApplicationController < ActionController::Base\n      end\n    end\n  end\nend\n/
-    assert_file "deep-hyphenated-name/app/models/deep/hyphenated/name/application_record.rb",          /module Deep\n  module Hyphenated\n    module Name\n      class ApplicationRecord < ActiveRecord::Base\n        primary_abstract_class\n      end\n    end\n  end\nend/
+    assert_file "deep-hyphenated-name/app/models/deep/hyphenated/name/application_record.rb",          /module Deep\n  module Hyphenated\n    module Name\n      class ApplicationRecord < ActiveRecord::Base\n        self.abstract_class = true\n      end\n    end\n  end\nend/
     assert_file "deep-hyphenated-name/app/jobs/deep/hyphenated/name/application_job.rb",               /module Deep\n  module Hyphenated\n    module Name\n      class ApplicationJob < ActiveJob::Base/
     assert_file "deep-hyphenated-name/app/mailers/deep/hyphenated/name/application_mailer.rb",         /module Deep\n  module Hyphenated\n    module Name\n      class ApplicationMailer < ActionMailer::Base\n        default from: "from@example\.com"\n        layout "mailer"\n      end\n    end\n  end\nend/
     assert_file "deep-hyphenated-name/app/helpers/deep/hyphenated/name/application_helper.rb",         /module Deep\n  module Hyphenated\n    module Name\n      module ApplicationHelper\n      end\n    end\n  end\nend/