Add green-blue-purple gradient color variant to Label component

[danielguillan]

Summary

Solves https://github.com/github/primer/issues/4583

List of notable changes:

• New green-blue-purple option available for Label color.

What should reviewers focus on?

• Gradient matches custom implementation in https://github.com/features/copilot
• See comment about custom colors in https://github.com/github/primer/issues/4583#issuecomment-2586989493

Steps to test:

• Storybook

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:

[changeset-bot]

🦋 Changeset detected

Latest commit: e2feac3

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

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]

🔍 Design token changes found

View CSS variable changes

+ --brand-Label-color-green-blue-purple-1: #000aff;

+ --brand-Label-color-green-blue-purple-2: #096bde;

+ --brand-Label-color-green-blue-purple-3: var(--base-color-scale-green-5);

+ --brand-Label-color-green-blue-purple-4: #8250df;

+ --brand-Label-color-green-blue-purple-3: #00ff46;

+ --brand-Label-color-green-blue-purple-4: #8250DF;

+ --brand-Label-color-green-blue-purple-3: var(--base-color-scale-green-5);

+ --brand-Label-color-green-blue-purple-2: #096BDE;

+ --brand-Label-color-green-blue-purple-1: #000AFF;

[github-actions]

🟢 No visual differences found

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

[joshfarrant]

🔥