Closes #25973 - add support for azurerm_application_gateway Basic SKU (Preview)

[tedsmitt]

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Change Log

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• azurerm_application_gateway - support for the Basic SKU tier

This is a (please select all that apply):

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

Related Issue(s)

Fixes #25973

[katbyte]

Looks like all the tests are failing with:

------- Stdout: -------
=== RUN   TestAccApplicationGateway_authCertificate
=== PAUSE TestAccApplicationGateway_authCertificate
=== CONT  TestAccApplicationGateway_authCertificate
    testcase.go:173: Step 1/6 error: Error running apply: exit status 1
        
        Error: creating Application Gateway (Subscription: "*******"
        Resource Group Name: "acctestRG-240920034903751201"
        Application Gateway Name: "acctestag-240920034903751201"): performing CreateOrUpdate: unexpected status 400 (400 Bad Request) with error: AppGatewayV1SkuDeprecated: Application Gateway SKU tier Standard has been deprecated. Supported SKU tiers are Standard_v2,WAF_v2. Refer https://aka.ms/V1retirement.
        
          with azurerm_application_gateway.test,
          on terraform_plugin_test.tf line 72, in resource "azurerm_application_gateway" "test":
          72: resource "azurerm_application_gateway" "test" {
        
--- FAIL: TestAccApplicationGateway_authCertificate (181.02s)
FAIL

[tedsmitt]

As per https://learn.microsoft.com/en-us/azure/application-gateway/v1-retirement#retirement-timelines it looks like we can no longer deploy V1 SKUs of the application gateway since the 1st Sept.

At the very least we need to remove any acceptance tests that use any V1 SKUs e.g Standard_Small, but the provider should be updated to remove these as valid options.

I believe that this is out of the scope of this PR, so we should raise another to remove this capability. I don't think this is too much effort. @katbyte shall another issue be raised for the removal of these deprecated SKUs?