Update to yarn latest

[garethbowen]

Description

Update to the latest yarn (4.x) from classic (1.x) we currently use. Migration guide: https://yarnpkg.com/migration/overview

There's nothing specifically blocking but keeps us up to date, and may get some benefits such as better performance out of the box.

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Checklist

• Does it need UI/UX design?
  • Yes. Describe the UI/UX requirements or attach a Figma link in the "Design" section.
  • No
• Does it need API design?
  • Yes. Specify the endpoints, methods, or data structures needed.
  • No
• Does it need design around state flow?
  • Yes. Outline the state flow requirements.
  • No
• Does it need a test plan?
  • Yes. Add the test plan as a comment in this task or attach the document.
  • No

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