chore(AvatarStack): Remove the CSS module feature flag from the AvatarStack component

[jonrohan]

Closes https://github.com/github/primer/issues/4024

Changelog

New

Changed

Removed

Remove the CSS module feature flag from the AvatarStack component

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: eee6906

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	94.43 KB (-0.88% 🔽)
packages/react/dist/browser.umd.js	94.47 KB (-0.88% 🔽)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/379051

[primer-integration]

🟢 golden-jobs completed with status success.

[Copilot]

Pull Request Overview

This PR removes the CSS module feature flag from the AvatarStack component, consolidating on a single implementation using BoxWithFallback and CSS modules exclusively.

• Eliminates all styled-components, feature-flag toggles, and legacy utilities from AvatarStack.
• Refactors AvatarStack to wrap with BoxWithFallback and always apply CSS module classes.
• Updates Jest snapshots for AvatarStack (and unexpectedly CircleBadge) to match the new class names and markup.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/react/src/CircleBadge/snapshots/CircleBadge.test.tsx.snap	Refreshed CSS module class names in CircleBadge snapshots
packages/react/src/AvatarStack/snapshots/AvatarStack.test.tsx.snap	Removed old CSS block, updated markup and class names for AvatarStack tests
packages/react/src/AvatarStack/AvatarStack.tsx	Stripped out feature-flag logic, styled components, and legacy utilities; introduced BoxWithFallback and direct CSS module usage
.changeset/loose-adults-thank.md	Added changeset noting removal of the CSS module feature flag

Comments suppressed due to low confidence (1)

packages/react/src/CircleBadge/snapshots/CircleBadge.test.tsx.snap:4

• [nitpick] The CircleBadge snapshot updates seem unrelated to the AvatarStack feature-flag removal. Consider isolating these snapshot changes into a separate PR or including an explanation to keep reviews focused.

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