Add entities files

ee/lib/ee/api/entities/merge_request_approval_setting_rule.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Entities
+      # Being used in private MR-level approvals API.
+      # This overrides the `eligible_approvers` to be exposed as `approvers`.
+      #
+      # To be removed in https://gitlab.com/gitlab-org/gitlab/issues/13574.
+      class MergeRequestApprovalSettingRule < MergeRequestApprovalStateRule
+        expose :approvers, using: ::API::Entities::UserBasic, override: true
+      end
+    end
+  end
+end

ee/lib/ee/api/entities/merge_request_approval_settings.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Entities
+      # Being used in private MR-level approvals API.
+      # This overrides the `rules` to be exposed using MergeRequestApprovalSettingRule.
+      #
+      # To be removed in https://gitlab.com/gitlab-org/gitlab/issues/13574.
+      class MergeRequestApprovalSettings < MergeRequestApprovalState
+        expose :wrapped_approval_rules, as: :rules, using: MergeRequestApprovalSettingRule, override: true
+      end
+    end
+  end
+end

ee/lib/ee/api/entities/project_approval_setting_rule.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Entities
+      # Being used in private project-level approvals API.
+      # This overrides the `eligible_approvers` to be exposed as `approvers`.
+      #
+      # To be removed in https://gitlab.com/gitlab-org/gitlab/issues/13574.
+      class ProjectApprovalSettingRule < ProjectApprovalRule
+        expose :approvers, using: ::API::Entities::UserBasic, override: true
+      end
+    end
+  end
+end

ee/lib/ee/api/entities/project_approval_settings.rb

+# frozen_string_literal: true
+
+module EE
+  module API
+    module Entities
+      # Being used in private project-level approvals API.
+      #
+      # To be removed in https://gitlab.com/gitlab-org/gitlab/issues/13574.
+      class ProjectApprovalSettings < Grape::Entity
+        expose :rules, using: ProjectApprovalSettingRule do |project, options|
+          project.visible_approval_rules(target_branch: options[:target_branch])
+        end
+
+        expose :min_fallback_approvals, as: :fallback_approvals_required
+      end
+    end
+  end
+end