Add warning about registering Microsoft.App provider

[gast0xC]

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Description

Added a warning in the azurerm_function_app_flex_consumption documentation to inform users that the Microsoft.App Resource Provider must be registered in their subscription before creating the resource.

Fixes #30418

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This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

[toddgiguere]

@gast0xC Thank you for the submission. We are going to close this PR without merge, as we do not usually document these types of back-end provider requirements, and they are much better explained in the official Azure documentation for the service.

We have added a way to let AzureRM users check for specific subscription provider registrations in the azurerm provider configuration in case these types of requirements are needed, documenation is here: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs#resource-provider-registrations