Fix color of temporary Duo agentic chat button icon

Purpose

The structure of the SVG makes it so that the fill="none" on the parent SVG tag prevents proper color changing in dark mode.

There is actually s subtle bug in light mode as well, the icon should not be literal #000.

This MR updates the paths to use currentColor instead so that they are responsive to the color of the SVG's parent.

Screenshots

Mode	Before	After
Light
Dark

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.