feat(br): Update border radius atomic styles

[ttaylor-stack]

Update border radius atomic styles as specified in the SPARK-91.

Ticket: https://stackoverflow.atlassian.net/browse/SPARK-91

Figma: https://www.figma.com/design/do4Ug0Yws8xCfRjHe9cJfZ/Project-SHINE---Product-UI?node-id=2213-7342&p=f&t=czT0zuZbbghSpKgs-0

Notes:

• Anywhere in the docs that used bar-sm or bar-lg I replaced with bar-md
• I left the base atomic styles that are used by the other components in place e.g. br-sm and br-md. I thought because it's used in so many components it makes sense to update them within the components story instead of this one.

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[dancormier]

Thank you for taking this on @ttaylor-stack ❤️

The tl;dr for my review is that we should get rid of all references of sm and lg border radius classes and custom properties. This extends to the custom property references in .less files across the code base.

Warning

Removing/replacing styles is likely to introduce merge conflicts for any in-flight component changes. I don't expect them to be tough to handle, but I figured it warrants a heads up.

[CGuindon]

I'm still seeing all the sm and lg classes in the documentation — does the preview need to be re-generated?

[dancormier]

For breaking changes, we should mention the breaks in the Migration Guide (example here). I think something general mentioning the removal of sm/lg border should be sufficient.

Besides that I think this is all set. We'll probably want to wait until the Avatar PR is merged before we merge this one, but we're close!

🙂 Thanks @ttaylor-stack