azurerm_service_plan - Support premium_site_elastic_scaling_enabled feature.

[xiaxyi]

Azure currently allows user to set the automatic scaling for premium v2 and v3 service plan, more detailed feature intro:https://learn.microsoft.com/en-us/azure/app-service/manage-automatic-scaling?tabs=azure-portal

MS 100% coverage task.

Acc tests:
--- PASS: TestAccServicePlan_premiumElasticScaling (154.22s)
PASS
ok github.com/hashicorp/terraform-provider-azurerm/internal/services/appservice 160.882s

The property is not designed for elastic plan, the value is always true and cannot be turned to false, as user is not able to set the property, remove from the state for elastic plan.

The failed CI doesn't relate to the code changed in this PR.

Failed acc tests are not related to the code change in this PR:

[jackofallops]

Closing due to conflicts, breaking change status and long open time. This can be re-opened when the issues/conflicts are resolved and is ready for re-review.

[github-actions]

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