neonvm: Make spec min/max/use resources required

[sharnoff]

This already was de-facto the case, which made much of the logic around handling "but what if .max is nil?" somewhat tricky to reason about.

Extracted from #970 and fleshed out here.

All our staging & production clusters have VMs meeting this criteria, with the notable exception of a single, already-broken VM in dev-us-east-2-beta (it's a long story).

More here: https://neondb.slack.com/archives/C03TN5G758R/p1718742308251569

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Checked each cluster with:

kubectl get neonvm -o json \
  |  jq -r '.items[] | select(.spec.guest | [.cpus.max,.cpus.min,.memorySlots.max,.memorySlots.min] | map(select(. == null)) | (length != 0)) | .metadata.name'