[Text] Update headingXs variant to use semibold instead of bold

[laurkim]

WHY are these changes introduced?

Resolves #7169.
Updates the headingXs variant to use semibold instead of bold.

WHAT is this pull request doing?

Updates the variant font mapping to use semibold for the headingXs variant.

Text headingXs variant before

Text headingXs variant after

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';

import {Page, Text} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
      <Text as="h6" variant="headingXs">
        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam ac
        convallis nulla, mollis sagittis est. Phasellus condimentum diam magna,
        et viverra sapien fringilla sed.
      </Text>
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[github-actions]

size-limit report 📦

Path	Size
polaris-react-cjs	202.08 KB (-0.01% 🔽)
polaris-react-esm	129.31 KB (-0.01% 🔽)
polaris-react-esnext	183.43 KB (-0.01% 🔽)
polaris-react-css	40.62 KB (0%)

[sarahill]

👏