[docs] reflect new operator-inventory options in the persistent storage docs

[andy108369]

operator-inventory helm-chart defaults to the beta2 storage type https://github.com/akash-network/helm-charts/blob/provider-9.1.2/charts/akash-inventory-operator/values.yaml#L13

We should update the docs in these regards https://akash.network/docs/providers/build-a-cloud-provider/helm-based-provider-persistent-storage-enablement/#inventory-operator

Basically just copy this chunk from https://akash.network/docs/other-resources/archived-resources/akash-provider-feature-discovery-upgrade-enablement/#akash-inventory-operator-installupgrade

And akash.network/storageclasses=<beta1|beta2|beta3> is not required anymore. The operator-inventory will label the nodes with akash.network/capabilities.storage.class.<beta1|beta2|beta3>=1 on its own.

[andy108369]

as per @chainzero :

This was added to documentation pre-feature discovery release as seen in GitBook below.
Appears we still have some documentation that was not carried over to the new doc site.

He'll follow-up

[andy108369]

• remove storageclasses label from the docs