Render upgrade notes as HTML instead of a pre block

The note is converted from Markdown with the markdown module, which is only
imported when a pull request has a note. HTML in the note is shown as text,
and link and image targets other than http(s), mailto and in-page are
replaced with "#".

Refs #9188

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>