Add default_project_deletion_protection to api

And api spec

ee/app/helpers/ee/application_settings_helper.rb

@@ -75,6 +75,7 @@ module EE
       repository_mirror_attributes + %i[
         email_additional_text
         file_template_project_id
+        default_project_deletion_protection
       ]
     end
   end

ee/lib/ee/api/entities.rb

@@ -158,6 +158,7 @@ module EE
           end)
           expose :email_additional_text, if: ->(_instance, _opts) { ::License.feature_available?(:email_additional_text) }
           expose :file_template_project_id, if: ->(_instance, _opts) { ::License.feature_available?(:custom_file_templates) }
+          expose :default_project_deletion_protection, if: ->(_instance, _opts) { ::License.feature_available?(:default_project_deletion_protection) }
         end
       end
 

ee/lib/ee/api/helpers/settings_helpers.rb

@@ -30,6 +30,7 @@ module EE
             end
 
             optional :email_additional_text, type: String, desc: 'Additional text added to the bottom of every email for legal/auditing/compliance reasons'
+            optional :default_project_deletion_protection, type: Grape::API::Boolean, desc: 'Disable project project owners ability to delete project'
             optional :help_text, type: String, desc: 'GitLab server administrator information'
             optional :repository_size_limit, type: Integer, desc: 'Size limit per repository (MB)'
             optional :file_template_project_id, type: Integer, desc: 'ID of project where instance-level file templates are stored.'

ee/spec/requests/api/settings_spec.rb

@@ -142,7 +142,7 @@ describe API::Settings, 'EE Settings' do
     it_behaves_like 'settings for licensed features'
   end
 
-  context 'mirroring settings' do
+  context 'default project deletion protection' do
     let(:settings) { { default_project_deletion_protection: true } }
     let(:feature) { :default_project_deletion_protection }