Update Progress Bar based on approved design

Design was approved in gitlab-org/gitlab-design#884 (closed) for the Progress Bar component, and we can update the Vue component based on the new design specs.

• View component in Figma →
• View component in Storybook →

Specifically, there are a couple of gaps between what's in Figma and code:

Note	Figma	Code
The border-radius in code should be fully-rounded, like in Figma. To set the height in code, instead of any value in pixels (default is 16px, which maps to Figma's md) it should have an array of sizes like in Figma — xs, sm, md.