Replace `<br>` with actual newline in notification message

While the Desktop Notifications Specification¹ does allow the body text to contain markup using a few HTML-based tags, <br> is not included in that list. As the specification clearly states: "notifications should never take advantage of tags that are not listed above".

¹ https://specifications.freedesktop.org/notification-spec/latest/index.html

BUG: 325753