Update Duo Chat breadcrumb button style

What does this MR do and why?

Based on #427634 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

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

1. Enable tanuki_bot_breadcrumbs_entry_point feature flag by running: Feature.enable(:tanuki_bot_breadcrumbs_entry_point)
2. Go to a page like a project or an issue
3. See the changed button design

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.