Update Duo Chat callout modal to include MRs

What does this MR do and why?

Updates the Duo Chat callout modal to include MRs

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

How to set up and validate locally

You can log in as someone who hasn't seen the callout modal before, or just use the rails console to destroy the callout for your user.

Example:

rails c

To reset for the default admin user:

user = User.first
callout = user.callouts.find_by(feature_name: 'duo_chat_callout')
callout.destroy!

Then, refresh the page of a group that has Duo Enterprise enabled.