Remove support for sx from BaseStyles component

[llastflowers]

Closes #6758

Current sx usage: 0

✅ Don't need to separately update github-ui components to use @primer/styled-react

Changelog

Removed

Remove support for sx from the BaseStyles component, and any associated stories, docs, and tests

Rollout strategy

• Major release; if selected, include a written rollout or migration plan

This component currently has 0 sx usage in dotcom, and therefore does not need the @primer/styled-react wrapper. this will be confirmed again before merging.

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: 9fb35f0

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/styled-react	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[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!

[Copilot]

Pull Request Overview

This PR removes support for the sx prop from the BaseStyles component as part of the migration away from styled-components. The change simplifies the component by removing system props handling and the associated wrapper component.

• Remove sx and system props support from BaseStyles component
• Update component implementation to use direct element rendering instead of BoxWithFallback
• Clean up associated tests, stories, and documentation

Reviewed Changes

Copilot reviewed 5 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/BaseStyles.tsx	Remove sx and system props support, simplify component implementation
packages/react/src/tests/BaseStyles.test.tsx	Remove test for system props functionality
packages/react/src/BaseStyles.dev.stories.tsx	Remove stories demonstrating sx and system props usage
e2e/components/BaseStyles.test.ts	Remove visual regression tests for sx and system props stories
.changeset/sharp-beans-run.md	Add changeset documenting the breaking change