Fix testimonial component color modes support

[danielguillan]

Summary

Adds missing color mode support to Testimonial component. Fixes #521

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: cf2d1d7

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	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

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[github-actions]

🟢 No design token changes found

[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences.

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[rezrah]

LGTM.

Could you please add a changeset before merging @danielguillan, otherwise good to go.