Refactor Reviewer Roulette Dangerfile

What does this merge request do and why?

This MR refactors the Dangerfile that drives the Reviewer Roulette.

• Use squiggly heredoc and remove extraneous lines
• Use format string syntax
• Remove support for category
• Reorder logic based on markdown content

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.