Fix Markdown attachments in Releases not rendering with full URL

When uploading an attachment to the release text, the relative URLs of
attachments were not converted to full URLs because the e-mail pipeline
wasn't being called.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/293966

app/views/notify/new_release_email.html.haml

@@ -15,4 +15,4 @@
 
   %p
     %h4= _("Release notes:")
-    = markdown_field(@release, :description)
+    = markdown(@release.description, pipeline: :email, author: @release.author)

changelogs/unreleased/sh-fix-release-markdown.yml

+---
+title: Fix Markdown attachments in Releases not rendering with full URL
+merge_request: 50146
+author:
+type: fixed

spec/mailers/emails/releases_spec.rb

@@ -49,5 +49,14 @@ RSpec.describe Emails::Releases do
       is_expected.to have_body_text('Release notes:')
       is_expected.to have_body_text(release.description)
     end
+
+    context 'release notes with attachment' do
+      let(:upload_path) { '/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg' }
+      let(:release) { create(:release, project: project, description: "Attachment: [Test file](#{upload_path})") }
+
+      it 'renders absolute links' do
+        is_expected.to have_body_text(%Q(<a href="#{project.web_url}#{upload_path}" data-link="true" class="gfm">Test file</a>))
+      end
+    end
   end
 end