Improve `AvatarStack` when hovering

[simurai]

What are you trying to accomplish?

This improves the AvatarStack when hovering.

What approach did you choose and why?

Adding border-radius: 100px; makes sure the background on hover doesn't have squares:

Before	After

What should reviewers focus on?

The AvatarStack could still be improved more, see https://github.com/github/primer/issues/122. But that is out of scope for this PR.

Can these changes ship as is?

• [x] Yes, this PR does not depend on additional changes. 🚢

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 3ffcfe0d854cf6d6a2593a04761db584e13e45d6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/css	Patch

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