[UnstyledButton] Add disabled attribute to UnstyledButton

[acmertz]

WHY are these changes introduced?

Fixes #7728

WHAT is this pull request doing?

Adds the disabled attribute to the <button> element that is rendered by UnstyledButton to prevent it from being selected by button:not(:disabled) CSS selectors.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

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

import React, {useState} from 'react';

import {Button, Modal, Page, Stack, TextField} from '../src';

export function Playground() {
  const [isModalOpen, setIsModalOpen] = useState(false);

  function handleClose() {
    setIsModalOpen(false);
  }

  function noop() {}

  return (
    <Page title="Playground">
      <Button onClick={() => setIsModalOpen(true)}>Open modal</Button>
      <Button disabled>Disabled button</Button>

      <Modal
        open={isModalOpen}
        onClose={handleClose}
        title="Example modal"
        primaryAction={{
          content: 'Primary action',
          onAction: handleClose,
          disabled: true,
        }}
      >
        <Modal.Section>
          <Stack vertical>
            <p>Some content</p>
            <TextField
              onChange={noop}
              autoComplete=""
              label="Text field 1"
              value=""
            />
            <TextField
              onChange={noop}
              autoComplete=""
              label="Text field 2"
              value=""
            />
          </Stack>
        </Modal.Section>
      </Modal>
    </Page>
  );
}

1. Copy the above code into Playground.tsx and run yarn dev
2. Confirm that the button labeled "Disabled button" has the disabled attribute
3. Confirm that this fixes [Modal] Focus isn't trapped in the modal when the primary action is disabled #7728:
   1. Click "Open modal"
   2. Use the Tab key to cycle through focusable elements within the modal
   3. Confirm that focus wraps around to the beginning of the modal despite the primary action button being disabled

🎩 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	210.4 KB (+0.01% 🔺)
polaris-react-esm	136.15 KB (+0.01% 🔺)
polaris-react-esnext	191.7 KB (+0.01% 🔺)
polaris-react-css	41.81 KB (0%)

[acmertz]

Closing in favor of #7760