[MediaCard] Fix the thumbnail’s corner roundness, so it won’t overflow out of the parent Card

[kaelig]

WHY are these changes introduced?

Fixes an issue in MediaCard where the thumbnail would overflow outside of the parent card on small screens at the top, as well as on larger screens, to the left, as in these screenshots:

WHAT is this pull request doing?

Uses the new design language custom properties instead of the Sass functions when applying border radius values.

How to 🎩

Compare new and old versions on Chromatic, at various viewport sizes.

🎩 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/MediaCard/MediaCard.tsx

📄 UNRELEASED.md (total: 0)

Files potentially affected (total: 0)

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

Files potentially affected (total: 1)

• /src/components/MediaCard/MediaCard.tsx

[alex-page]

👏 Amazing thanks Kaelig!