use default tab order for AnchorNav

[joshfarrant]

Summary

Replace arrow key navigation with standard tab order navigation for the AnchorNav component.

What should reviewers focus on?

• Ensure you can navigate through an AnchorNav using Tab and Shift + Tab keys

Steps to test:

1. Open the AnchorNav component in Storybook
2. Resize your viewport to see the mobile AnchorNav
3. Use the Tab key to navigate through the links
4. Use the Shift + Tab to navigate backwards

Supporting resources (related issues, external links, etc):

• Link to updated AnchorNav Storybook
• https://github.com/github/accessibility-audits/issues/7852

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
• New visual snapshots have been generated / updated for any UI changes
• 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:

7852.mp4

[changeset-bot]

🦋 Changeset detected

Latest commit: 9acb271

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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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.

[stamat]

Awesome! ✨ Great work of killing these customisations! We should also consifer shipping this #595 🙏