Convert jira issue dates to UTC on serialization

ee/app/serializers/integrations/jira/issue_entity.rb

@@ -12,15 +12,15 @@ module Integrations
       end
 
       expose :created_at do |jira_issue|
-        jira_issue.created
+        jira_issue.created.to_datetime.utc
       end
 
       expose :updated_at do |jira_issue|
-        jira_issue.updated
+        jira_issue.updated.to_datetime.utc
       end
 
       expose :closed_at do |jira_issue|
-        jira_issue.resolutiondate
+        jira_issue.resolutiondate&.to_datetime&.utc
       end
 
       expose :labels do |jira_issue|

ee/spec/serializers/integrations/jira/issue_entity_spec.rb

@@ -25,9 +25,9 @@ RSpec.describe Integrations::Jira::IssueEntity do
     expect(subject).to include(
       project_id: project.id,
       title: 'summary',
-      created_at: '2020-06-25T15:39:30.000+0000',
-      updated_at: '2020-06-26T15:38:32.000+0000',
-      closed_at: '2020-06-27T13:23:51.000+0000',
+      created_at: '2020-06-25T15:39:30.000+0000'.to_datetime.utc,
+      updated_at: '2020-06-26T15:38:32.000+0000'.to_datetime.utc,
+      closed_at: '2020-06-27T13:23:51.000+0000'.to_datetime.utc,
       labels: [
         {
           name: 'backend',