Make clear select button match design spec

[mollykreis]

Pull Request

🤨 Rationale

The clear button in the nimble-select deviated from the design spec in two ways:

• The button was 18px by 18px rather than 24px by 24px
• The icon in the button was nimble-icon-xmark rather than nimble-icon-times

👩‍💻 Implementation

• Update icon and button size

🧪 Testing

Manually tested 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?

[rajsite]

bypassing @jattasNI straightforward sizing and icon switch