Prevent premailer from converting HTML entities

This prevents `&mdash` in our templates being change to `-`

changelogs/unreleased/55964-fix-email-encoding.yml

+---
+title: Fix notfication emails having wrong encoding
+merge_request: 26931
+author:
+type: fixed

config/initializers/premailer.rb

@@ -4,5 +4,6 @@ Premailer::Rails.config.merge!(
   preserve_styles: true,
   remove_comments: true,
   remove_ids: false,
-  remove_scripts: false
+  remove_scripts: false,
+  output_encoding: 'US-ASCII'
 )

spec/mailers/notify_spec.rb

@@ -30,6 +30,19 @@ describe Notify do
                    description: 'My awesome description!')
   end
 
+  describe 'with HTML-encoded entities' do
+    before do
+      described_class.test_email('test@test.com', 'Subject', 'Some body with —').deliver
+    end
+
+    subject { ActionMailer::Base.deliveries.last }
+
+    it 'retains 7bit encoding' do
+      expect(subject.body.ascii_only?).to eq(true)
+      expect(subject.body.encoding).to eq('7bit')
+    end
+  end
+
   context 'for a project' do
     shared_examples 'an assignee email' do
       it 'is sent to the assignee as the author' do