Fix issue due notification emails threading

It should not be a start to a new thread but rather
a reply to an existing thread

app/mailers/emails/issues.rb

@@ -11,7 +11,7 @@ module Emails
     def issue_due_email(recipient_id, issue_id, reason = nil)
       setup_issue_mail(issue_id, recipient_id)
 
-      mail_new_thread(@issue, issue_thread_options(@issue.author_id, recipient_id, reason))
+      mail_answer_thread(@issue, issue_thread_options(@issue.author_id, recipient_id, reason))
     end
 
     def new_mention_in_issue_email(recipient_id, issue_id, updated_by_user_id, reason = nil)

changelogs/unreleased/66524-issue-due-notification-emails-are-threaded-incorrectly.yml

+---
+title: Fix issue due notification emails not being threaded correctly
+merge_request: 32325
+author:
+type: fixed

spec/mailers/notify_spec.rb

@@ -187,6 +187,22 @@ describe Notify do
         end
       end
 
+      describe 'that are due soon' do
+        subject { described_class.issue_due_email(recipient.id, issue.id) }
+
+        before do
+          issue.update(due_date: Date.tomorrow)
+        end
+
+        it_behaves_like 'an answer to an existing thread with reply-by-email enabled' do
+          let(:model) { issue }
+        end
+        it_behaves_like 'it should show Gmail Actions View Issue link'
+        it_behaves_like 'an unsubscribeable thread'
+        it_behaves_like 'appearance header and footer enabled'
+        it_behaves_like 'appearance header and footer not enabled'
+      end
+
       describe 'status changed' do
         let(:status) { 'closed' }
         subject { described_class.issue_status_changed_email(recipient.id, issue.id, status, current_user.id) }