Fix mismatched borderRadius token name between Figma and npm

[mkernohanbc]

We have a mismatch between our Figma library and our code library in the name of this token:

• Figma: layout.borderRadius.default
• CSS: --layout-border-radius-medium

The final part of the token name was changed from medium to default in 3.0.0, based on feedback from user testing that suggested people were instinctively looking for a 'correct', 'standard' value.

Best guess at what happened here is that that change happened in Figma after the Tokens Studio export used to create the 3.0.0 npm build. The value is set to medium in the tokens.json output file: https://github.com/ty2k/design-system/blob/1f81f09aefe592bf9531f2419ca7865e3c2e6b64/packages/design-tokens/input/tokens.json#L461

In the next release of the design tokens package, we need to re-match these two tokens. We have two options:

• Change CSS/JS token to default: breaking change
• Revert Figma token to medium and provide clarity another way (eg. through token descriptions - see Add descriptions to tokens in Figma #350)

[Philip-Cheung]

The easiest thing to do is to revert it back to medium. I won't be able to do it on TS because of that github link credentials bug (Tyler will need to unlink it again for me to do it). In the meantime, I can add a description on the token to say medium is the default radius.