azurerm_kubernetes_cluster: Support migration of service_principal to identity

[aristosvo]

Fixes #11288

TO DO

• Cover edge cases
• Docs
• Run tests (only servicePrincipal tests)

Acceptance Tests:

• TestAccKubernetesCluster_servicePrincipal -> FAIL because I don't have a secondary principal specified
• TestAccKubernetesCluster_servicePrincipalToSystemAssignedIdentity -> PASS
• TestAccKubernetesCluster_servicePrincipalToUserAssignedIdentity -> PASS

[aristosvo]

@mbfrahry Another AKS upgrade 😄

I hope you don't mind, would you be able to run the whole AccTests suite? I don't have the resources at the moment.

[mbfrahry]

This is looking good! I just have some suggestions about presenting the info in the docs

[mbfrahry]

LGTM!

[ghost]

This has been released in version 2.63.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.63.0"
}
# ... other configuration ...

[github-actions]

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