refactor: migrate from `slice.items` to `slice.primary` groups

[lihbr]

Contributes to: DT-2080

Context

This PR migrates slices from using slice.items array to using regular group fields within the slice.primary section.

Before Merging

• [x] Update content dump (WIP Michel)
• [x] Update repositories on production (once backend is on production)
• [x] Revert slicemachine.config.json

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