azurerm_monitor_action_group - fix method to check exactly one of event_hub_id or (event_hub_namespace, event_hub_name) must be set in event_hub_receiver

[teowa]

In #17335 the event_hub_id is deprecated in favor of event_hub_namespace, event_hub_name, all of the 3 field is marked as optional and computed. But there is bug to check the exactly one of of these properties. This PR uses the d.GetRawConfig() to check the exactly one of.

test:

TF_ACC=1 go test -v ./internal/services/monitor -parallel 20 -test.run=TestAccMonitorActionGroup_event -timeout 1440m -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccMonitorActionGroup_eventHubReceiver
=== PAUSE TestAccMonitorActionGroup_eventHubReceiver
=== CONT  TestAccMonitorActionGroup_eventHubReceiver
--- PASS: TestAccMonitorActionGroup_eventHubReceiver (283.59s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/monitor       283.629s

resolves #20146

[mbfrahry]

LGTM! Thanks @teowa

[github-actions]

This functionality has been released in v3.56.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

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[github-actions]

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