Fix text field start slot color

[mollykreis]

Pull Request

🤨 Rationale

An icon put in the start slot of the nimble-text-field had the incorrect color based on the visual design spec. Specifically, the icon had been the text color, but it should have a 60% opacity applied to it.

👩‍💻 Implementation

Updated the styling of the start slot

🧪 Testing

Manually tested across all 3 themes in storybook

✅ Checklist

• [ ] I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@atmgrifter00, will you buddy this PR for me?