azurerm_eventhub_namespace: fix downgrade scenario

[favoretti]

Fixes #10244.

In a corner case when we are downgrading from Standard to Basic SKU and namespace had both autoInflate enabled and
maximumThroughputUnits set - we need to force throughput units back to 0, otherwise downgrade fails

$ TF_ACC=1 go test -v ./azurerm/internal/services/eventhub -timeout=1000m -run=TestAccEventHubNamespace_TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput -run=TestAccEventHubNamespace_basicWithIdentity
=== RUN   TestAccEventHubNamespace_basicWithIdentity
=== PAUSE TestAccEventHubNamespace_basicWithIdentity
=== CONT  TestAccEventHubNamespace_basicWithIdentity
--- PASS: TestAccEventHubNamespace_basicWithIdentity (269.03s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/eventhub	270.272s
$ TF_ACC=1 go test -v ./azurerm/internal/services/eventhub -timeout=1000m -run=TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput
=== RUN   TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput
=== PAUSE TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput
=== CONT  TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput
--- PASS: TestAccEventHubNamespace_SkuDowngradeFromAutoInflateWithMaxThroughput (678.40s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/eventhub	679.681s

[katbyte]

Thanks @favoretti - LGTM 👍

[ghost]

This has been released in version 2.47.0 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 = "~> 2.47.0"
}
# ... other configuration ...

[ghost]

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