HdsSideNav deprecations

[zamoore]

📌 Summary

If merged, this PR removes several deprecated features of the HdsSideNav component.

🛠️ Detailed description

• Removed the @ariaLabel argument, its usage in the index component, and its associated tests.
• Removed the HdsSideNavHeaderIconButton component and its docs and tests.
• Pushes the removal target for the HdsSideNav component from 5.0.0 to 6.0.0

🔗 External links

Jira ticket: HDS-5515

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👀 Component checklist

• Percy was checked for any visual regression
• A changelog entry was added via Changesets if needed (see templates here)

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

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