Decrease default heading size in River

[rezrah]

Summary

Closes https://github.com/github/primer/issues/4402

Also removed CRA test, due to lack of OSS support and low framework adoption.

🔗 Preview

List of notable changes:

• Decreases the default heading size value in River from 4 to 5.

What should reviewers focus on?

• Review the diffs for design correctness

Steps to test:

1. Go the the diffs in 'files changed' tab
2. Review the before and after visual snapshots
3. Review the Storybook examples

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: 39a8b6c

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

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	Minor
@primer/brand-primitives	Minor
@primer/brand-e2e	Minor
@primer/brand-fonts	Minor
@primer/brand-config	Minor
@primer/brand-storybook	Minor

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

[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[joshfarrant]

LGTM!

[danielguillan]

Looks good!

[natalie-iv]

looks good!