[AVM Module Issue]: Remove GPv1 Storage Account Support from AVM Storage Modules Following Azure GPv1 Retirement

### Check for previous/existing GitHub issues

- [x] I have checked for previous/existing GitHub issues

### Issue Type?

Feature Request

### Module Name

avm/res/storage/storage-account

### (Optional) Module Version

_No response_

### Description

Azure Storage is retiring General Purpose v1 (GPv1) storage accounts, with automatic migrations beginning after the retirement milestone 2026. https://learn.microsoft.com/en-us/azure/storage/common/general-purpose-version-1-account-migration-overview

The Azure Verified Modules (AVM) Storage Account modules (Bicep and Terraform) already default to GPv2 account types. However, they currently continue to expose GPv1 account kinds/SKUs as supported values to maintain backwards compatibility with existing customer deployments.

Once the GPv1 retirement process has completed and Azure no longer supports GPv1 storage accounts, AVM should remove GPv1-related values from module inputs and validation rules.

|Date | Milestone|
|-- | --|
|September 2025 | Retirement announced|
|September 2026 | Creation of new GPv1 Storage accounts disabled|
|October 2026 | Full retirement; Any remaining GPv1 Storage accounts will be automigrated to GPv2. Your decision not to migrate an existing GPv1 account will be construed as consent for Microsoft to migrate the account on your behalf.|
|November 2026 onwards| AVM should do the work|

> [!NOTE]
> Child of https://msft.ghe.com/azure-cloud-native/Azure-Verified-Modules/issues/189

### (Optional) Correlation Id

_No response_