fix: ensure Duo Chat conversation is scrolled to bottom on load

Description

fix: ensure Duo Chat conversation is scrolled to bottom on load

Previous scrolling solution uses custom setTimeout/window.scrollTo, which did not work for the GlDuoChat component, because scrolling was within an internal container rather than the full document.

Replaced with a call to the GlDuoChat components existing scrollToBottom method, which correctly sets scroll position for the internal container and handles displaying/hiding the fade overlay element.

This fix relies on an update in @gitlab/ui and should not be merged until the related MR is merged, and gitlab/ui is updated within this project. This is now updated via !1889 (merged)

Related Issues

Resolves #1398 (closed)

How has this been tested?

• Open the Duo Chat panel, and get enough chat history to allow scrolling, e.g. prompt Explain GitLab in detail
• Close the Duo Chat panel.
• Open it again.
• Verify chat has opened with scroll position correctly set to the bottom (bottom of most recent message visible)

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes

Closes #1398 (closed)