Rename milestone collection resolvers to plural

Since these return a collection of milestones, we pluralize the name.
We do this since we'll be adding another resolver for single
milestones.

app/graphql/resolvers/group_milestone_resolver.rb → app/graphql/resolvers/group_milestones_resolver.rb

 # frozen_string_literal: true
 
 module Resolvers
-  class GroupMilestoneResolver < MilestoneResolver
+  class GroupMilestonesResolver < MilestonesResolver
     argument :include_descendants, GraphQL::BOOLEAN_TYPE,
              required: false,
              description: 'Also return milestones in all subgroups and subprojects'

app/graphql/resolvers/milestone_resolver.rb → app/graphql/resolvers/milestones_resolver.rb

 # frozen_string_literal: true
 
 module Resolvers
-  class MilestoneResolver < BaseResolver
+  class MilestonesResolver < BaseResolver
     include Gitlab::Graphql::Authorize::AuthorizeResource
     include TimeFrameArguments
 
@@ -35,7 +35,7 @@ module Resolvers
     end
 
     def parent
-      @parent ||= object.respond_to?(:sync) ? object.sync : object
+      synchronized_object
     end
 
     def parent_id_parameters(args)

app/graphql/resolvers/project_milestone_resolver.rb → app/graphql/resolvers/project_milestones_resolver.rb

 # frozen_string_literal: true
 
 module Resolvers
-  class ProjectMilestoneResolver < MilestoneResolver
+  class ProjectMilestonesResolver < MilestonesResolver
     argument :include_ancestors, GraphQL::BOOLEAN_TYPE,
              required: false,
              description: "Also return milestones in the project's parent group and its ancestors"

app/graphql/types/group_type.rb

@@ -51,7 +51,7 @@ module Types
 
     field :milestones, Types::MilestoneType.connection_type, null: true,
           description: 'Milestones of the group',
-          resolver: Resolvers::GroupMilestoneResolver
+          resolver: Resolvers::GroupMilestonesResolver
 
     field :boards,
           Types::BoardType.connection_type,

app/graphql/types/project_type.rb

@@ -150,7 +150,7 @@ module Types
 
     field :milestones, Types::MilestoneType.connection_type, null: true,
           description: 'Milestones of the project',
-          resolver: Resolvers::ProjectMilestoneResolver
+          resolver: Resolvers::ProjectMilestonesResolver
 
     field :project_members,
           Types::ProjectMemberType.connection_type,

spec/graphql/resolvers/group_milestone_resolver_spec.rb → spec/graphql/resolvers/group_milestones_resolver_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Resolvers::GroupMilestoneResolver do
+RSpec.describe Resolvers::GroupMilestonesResolver do
   include GraphqlHelpers
 
   describe '#resolve' do

spec/graphql/resolvers/project_milestone_resolver_spec.rb → spec/graphql/resolvers/project_milestones_resolver_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Resolvers::ProjectMilestoneResolver do
+RSpec.describe Resolvers::ProjectMilestonesResolver do
   include GraphqlHelpers
 
   describe '#resolve' do