Don't expand the limayaml structure with private fields

[jandubois]

Lima is going to switch to strict YAML validation, refusing lima.yaml files that contain unknown keys.

Rancher Desktop stores the k3s version in a custom field:

rancher-desktop/src/backend/lima.ts

Lines 114 to 117 in c7ea750

	// The rest of the keys are not used by lima, just state we keep with the VM.
	k3s?: {
	version: string;
	}

This should be moved to either an env variable, or to a provision script.

The custom field should be removed from existing instances during Rancher Desktop upgrade.