Migrate messages icon to bullhorn

What does this MR do and why?

Migrate messages icon to bullhorn since messages is now deprecated and will be removed from the library.

messages and bullhorn are equal. We're removing duplicated icons.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to http://gdk.test:3000/admin/broadcast_messages
2. Look at the active nav bar item
3. Click [add new message]
4. Look at the example message

#452231 (closed)