feat(Heading): remove support for sx prop

[joshblack]

Closes #6764

Changelog

New

Changed

Removed

• Remove support for sx from Heading
• Remove support for sx from PageHeader.Title

Rollout strategy

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

We are using the @primer/styled-react migration method for sx for this component

[changeset-bot]

🦋 Changeset detected

Latest commit: c6400b2

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

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[Copilot]

Pull Request Overview

This PR removes support for the sx prop from the Heading component and PageHeader.Title as part of the migration away from styled-components to CSS modules. The refactor introduces a styled-react compatibility layer for consumers still using the sx prop.

Key changes:

• Remove sx prop support from the core Heading component implementation
• Add compatibility wrapper in @primer/styled-react package for gradual migration
• Clean up related tests, stories, and documentation that used the deprecated sx prop

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/Heading/Heading.tsx	Remove sx prop support and Box wrapper from core component
packages/styled-react/src/components/Heading.tsx	Add compatibility wrapper that preserves sx functionality via Box
packages/react/src/PageHeader/PageHeader.tsx	Remove sx prop from PageHeader.Title component
packages/react/src/Heading/__tests__/Heading.test.tsx	Remove tests for deprecated sx functionality
packages/react/src/Heading/Heading.features.stories.tsx	Remove story demonstrating sx usage
packages/react/src/Popover/Popover.figma.tsx	Update Figma example to remove sx usage

[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!

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/3166

[primer-integration]

🟢 ci completed with status success.