Swapped AIGenieChat with DuoChat component

What does this MR do and why?

The MR integrates the shared GlDuoChat component into GitLab instead of the AIGenieChat. In essence, the switch should be seamless, except for some minor visual and UX changes contributed to the shared component in the context of gitlab-ui project.

Screenshots or screen recordings

Before	After

⚠ Note the absence of the icon in the header of the chat in the shared component - it's by design and approved by ~UX.

How to set up and validate locally

• Set up GitLab Duo Chat. Mainly, you're interested in running the automatic script
• Start the chat from the left-hand navigation panel in "Help -> GitLab Duo Chat"
• Have fun

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.

Related to #422707 (closed), and #422708 (closed)