az functionapp plan create fails when specifying Elastic Premium "--sku EP1" in East US 2

## Describe the bug

**Command Name**
`az functionapp create`

**Errors:**
```
usage error: Currently supported runtimes (--runtime) in windows function apps are: dotnet, node, custom, java, powershell.
```

## To Reproduce:

Create a Python Function App using "[az functionapp create](https://docs.microsoft.com/en-us/cli/azure/functionapp?view=azure-cli-latest#az_functionapp_create)" on a Premium plan created with "[az functionapp plan create](https://docs.microsoft.com/en-us/cli/azure/functionapp/plan?view=azure-cli-latest#az_functionapp_plan_create)":

```shell
sub_id={ subscription ID }
rg_name={ resource group name }
sa_name={ storage account name }
asp_name={ app service plan name }
func_name={ function app name }

az functionapp plan create \
--subscription $sub_id \
--resource-group $rg_name \
--name $asp_name \
--location eastus2 \
--sku EP1

az functionapp create \
--subscription $sub_id \
--resource-group $rg_name \
--storage-account $sa_name \
--plan $asp_name \
--name $func_name \
--functions-version 3 \
--os-type Linux \
--runtime python \
--runtime-version 3.8
```

## Expected Behavior

Create a Linux hosted Function app with the requested Python runtime.

## Environment Summary
```
Linux-5.4.0-56-generic-x86_64-with-debian-bullseye-sid
Python 3.6.10
Installer: DEB

azure-cli 2.16.0

Extensions:
azure-devops 0.18.0
```

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: e2f254bb-7dfd-417b-b08d-f2ac34247d99
* Version Independent ID: 9d22f523-579b-ea66-04ed-20f09c72bcbb
* Content: [az functionapp](https://docs.microsoft.com/en-us/cli/azure/functionapp?view=azure-cli-latest#az-functionapp-create)
* Content Source: [latest/docs-ref-autogen/functionapp.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/functionapp.yml)
* Service: **functions**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**