Fix border on Windows high contrast

[dleroux]

WHY are these changes introduced?

I noticed this while working on slides, so I fixed it. The shadow no longer has an impact on Chromium edge and Windows high contrast mode.

WHAT is this pull request doing?

Adds an outline to Banner.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

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

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </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
• For visual design changes, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[github-actions]

🟢 This pull request modifies 2 files and might impact 1 other files.

Details:

All files potentially affected (total: 1)

• /src/components/Banner/Banner.tsx

📄 UNRELEASED.md (total: 0)

Files potentially affected (total: 0)

🎨 src/components/Banner/Banner.scss (total: 1)

Files potentially affected (total: 1)

• /src/components/Banner/Banner.tsx

[AndrewMusgrave]

Didn't tophat but looks good to me