Validate policy mode value

[beandrad]

Policy creation succeeds even if the policy mode in the request is
not capitalized, however, when fetching the policy mode value from
Azure, the correct capitalized version is returned, which causes
the Terraform state and configuration to be different.

Ideally, there should be an enum with a set of allowed values, but
thi is not the case in the latest version of the Azure Rest API specs.
However, the supported mode values are returned in the create error
response when an invalid mode is requested.

Fixes #8748

[beandrad]

[tombuildsstuff]

hey @beandrad

Thanks for this PR - if we can also update the docs then this otherwise LGTM 👍

Thanks!

[tombuildsstuff]

could we also update the values used in the docs here: https://github.com/terraform-providers/terraform-provider-azurerm/blame/master/website/docs/r/policy_definition.html.markdown#L72-L74

* `mode` - (Required) The policy mode that allows you to specify which resource types will be evaluated. Possible values are `All`, `Indexed`, `Microsoft.ContainerService.Data`, `Microsoft.CustomerLockbox.Data`, `Microsoft.DataCatalog.Data`, `Microsoft.KeyVault.Data`, `Microsoft.Kubernetes.Data`, `Microsoft.MachineLearningServices.Data`, `Microsoft.Network.Data` and `Microsoft.Synapse.Data`.

[Nepomuceno]

I do think that a DiffSupress Would be better for this them forcing the values.

[beandrad]

@tombuildsstuff thanks for reviewing! docs updated!

[ghost]

This has been released in version 2.31.0 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 = "~> 2.31.0"
}
# ... other configuration ...

[ghost]

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