Breaking Behavioural Change to the AKS API

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It appears there's been a breaking behavioural change to the response for AKS API version `2019-06-01` where the sku returned is now "Standard" rather than "standard".

Whilst I'm generally supportive of this change to keep the casing consistent with the other resources - given this breaks the behaviour of the API shouldn't this be made in a new API version?

Thanks!

Original issue: https://github.com/terraform-providers/terraform-provider-azurerm/issues/5007#issuecomment-561149598

cc @jluk

Review the AKS API version 2019-06-01 specification and the linked original Terraform issue, focusing on the reported change from "standard" to "Standard". Determine whether the casing change is a breaking contract change and whether it requires a new API version; done means the versioning decision is documented or routed to the appropriate specification change.

Written by the indexing model from the issue text.