Make Sidekiq warnings more visible in danger output

What does this MR do and why?

This MR makes Sidekiq warnings more visible in the Danger Bot output by adding a warning to the top-level comment, as well as expanding on the reasoning for the in-line comment.

This is a response to a previous issue where the in-line comment was prematurely resolved.

Related issue: #425692 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Update the method signature of a Sidekiq worker's perform method and create a commit.
2. Run bin/rake danger_local in your terminal.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.