azurerm_kubernetes_cluster - do not pass the maintenance window start date if it's not changed

[ms-henglu]

fixes #22762

In the AKS maintenance window block, the start_date is an optional+computed field, if it's not set, the server will return the current date as a default value.

When update the maintenance window block, the start_date previous default value could be invalid because the server will check whether the start_date is before current date.

This PR compares the start_date's value with the value from server side, and only set it in the update when they're different.

=== RUN   TestAccKubernetesCluster_basicMaintenanceConfigAutoUpgrade
=== PAUSE TestAccKubernetesCluster_basicMaintenanceConfigAutoUpgrade
=== CONT  TestAccKubernetesCluster_basicMaintenanceConfigAutoUpgrade
--- PASS: TestAccKubernetesCluster_basicMaintenanceConfigAutoUpgrade (688.73s)
=== RUN   TestAccKubernetesCluster_updateMaintenanceConfig
=== PAUSE TestAccKubernetesCluster_updateMaintenanceConfig
=== CONT  TestAccKubernetesCluster_updateMaintenanceConfig
--- PASS: TestAccKubernetesCluster_updateMaintenanceConfig (1134.48s)

[stephybun]

Instead of passing in nil here could you please split the expand into two separate functions, one for create and one for update so it's clear at first glance that they are being handled differently?

[ms-henglu]

Thanks @stephybun ! I've updated this PR as suggested.

[stephybun]

Thanks @ms-henglu but we have a test failure:

------- Stdout: -------
=== RUN   TestAccKubernetesCluster_updateMaintenanceConfig
=== PAUSE TestAccKubernetesCluster_updateMaintenanceConfig
=== CONT  TestAccKubernetesCluster_updateMaintenanceConfig
    testcase.go:113: Step 9/18 error: Error running apply: exit status 1
        Error: creating/updating Auto Upgrade Schedule Maintenance Configuration for Kubernetes Cluster (Subscription: "*******"
        Resource Group Name: "acctestRG-aks-231130074301786320"
        Kubernetes Cluster Name: "acctestaks231130074301786320"): maintenanceconfigurations.MaintenanceConfigurationsClient#CreateOrUpdate: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InvalidParameter" Message="The input 'maintenanceWindow.startDate' 2023-11-26 00:00:00 +0100 cxTimeZone is before the current time 2023-11-30 07:49:00.745714287 +0000 UTC m=+684224.837081946."
          with azurerm_kubernetes_cluster.test,
          on terraform_plugin_test.tf line 26, in resource "azurerm_kubernetes_cluster" "test":
          26: resource "azurerm_kubernetes_cluster" "test" {
--- FAIL: TestAccKubernetesCluster_updateMaintenanceConfig (683.67s)
FAIL

[ms-henglu]

Thanks @stephybun , I've updated the test case.

=== RUN   TestAccKubernetesCluster_updateMaintenanceConfig
=== PAUSE TestAccKubernetesCluster_updateMaintenanceConfig
=== CONT  TestAccKubernetesCluster_updateMaintenanceConfig
--- PASS: TestAccKubernetesCluster_updateMaintenanceConfig (1277.83s)
PASS

[stephybun]

@ms-henglu lint?

[stephybun]

Thanks, LGTM 👍

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