Change default series icon shape to circle

[envex]

What does this implement/fix?

As part of https://github.com/Shopify/analytics-ui-components/pull/322 we want to change the default series icon to a circle instead of a rounded square.

Figma

What do the changes look like?

Storybook link

Before merging

• Check your changes on a variety of browsers and devices.

• Update the Changelog's Unreleased section with your changes.

• Update relevant documentation, tests, and Storybook.

• Make sure you're exporting any new shared Components, Types and Utilities from the top level index file of the package

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
polaris-viz-core-cjs	61.75 KB (0%)	1.3 s (0%)	1.2 s (+8.15% 🔺)	2.4 s
polaris-viz-cjs	230.46 KB (+0.06% 🔺)	4.7 s (+0.06% 🔺)	2.3 s (+20.41% 🔺)	6.9 s
polaris-viz-esm	186.33 KB (+0.03% 🔺)	3.8 s (+0.03% 🔺)	1.9 s (+32.07% 🔺)	5.6 s
polaris-viz-css	5.86 KB (+0.37% 🔺)	118 ms (+0.37% 🔺)	488 ms (+24.82% 🔺)	606 ms
polaris-viz-esnext	193.62 KB (+0.04% 🔺)	3.9 s (+0.04% 🔺)	1.3 s (+6% 🔺)	5.2 s

[michaelnesen]

LGTM 👍