feat: add new chat icon to activity bar

Description

This MR:

• Updates the activity bar to use the new Duo Chat icon.
• Updates the Duo Chat icon to use 1px stroke to align with VS Code icon standards.
• Deletes the old Duo icon as it's no longer used.

Related Issues

Resolves #1514 (closed)

How has this been tested?

• Locally run extension
• View new icon in the activity bar (left). Test selected/unselected.
• Click Duo in status bar to view in quick pick menu. Test with both Duo Chat and Code Suggestions enabled/disabled.

Screenshots (if appropriate)

Activity bar	Status menu

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 #1514 (closed)