fix: Update SidePanel to use sidebarLeft icon

[alanbsmith]

Summary

Fixes: #4069

• Bump canvas-system-icons-web to v4.2.0 (we didn't need to do this, but I went ahead with it to keep us current).
• Update SidePanel to use sidebarLeftIcon

The SidePanel component doesn't match our latest v15 Figma, so we're updating it.

Release Category

Components

Release Note

This update will break any visual snapshots as the icon has changed from extendIcon to sidebarLeftIcon. It is safe to accept these visual changes. The icon also no longer flips on expand / collapse, but it will still respect end (right) origin and RTL.

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• MDX documentation adheres to Canvas Kit's Documentation Guidelines
• Label ready for review has been added to PR

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