[Engineering] What's New - Update Danger Bot for What's New YAML

Overview

With the work in #5385 (closed) we introduced the concept of using a YAML file to generate the content for the What's New feature. Danger bot doesn't know what to do with the files, however, so it doesn't assign them automatically to a reviewer. This can be seen in gitlab-org/gitlab!40123 (comment 400117280) :

"These files couldn't be categorised, so Danger was unable to suggest a reviewer. Please consider creating a merge request to add support for them."

Proposal

Add support to danger bot for these files so it knows who to assign as a reviewer.