When originally introduced, the field was made immutable to be
consistent with the other fields. But in practice allowing it to be
toggled makes more sense, in particular when considering the rollout
of a CSI driver (let it run without using the published
CSIStorageCapacity object, then flip the field, or upgrading from a
driver without support to one which supports it).
The only consumer of this field, the kube-scheduler, can handle
mutation without problems because it always consults the informer
cache to get the current value.
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