CPU Arch now lives under host.cpu #2374

[thompson-tomo]

Fixes #2374

Changes

This moves the CPU arch to now be under the host.cpu namespace rather than directly under the host namespace

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Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
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• [x] schema-next.yaml updated with changes to existing conventions.