[API Regression] [Purview] Accounts Sku Capacity is always reset to 1

It appears that a regression has been rolled out to the Purview API's (both 2020-12-01-preview and 2021-07-01) where any valid value for `capacity` within `sku` (either 4 or 16) is reset to 1.

This applies both through the API and the Portal, where regardless of whether `capacity` is set to `4` or `16`, the value of `1` is returned:

Viewing the result of a Purview Account created with either `4` or `16` returns the following:

> ID: `/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-devrg/providers/Microsoft.Purview/accounts/tomdevpurview22`

```json
{
"sku": {
"name": "Standard",
"capacity": 1
},
"properties": {
"cloudConnectors": {
"awsExternalId": "00000000-0000-0000-0000-000000000000"
},
"friendlyName": "tomdevpurview22",
"createdBy": "redacted@somedomain.com",
"createdByObjectId": "00000000-0000-0000-0000-000000000000",
"createdAt": "2021-08-17T10:26:06.8974892Z",
"endpoints": {
"catalog": "https://tomdevpurview22.catalog.purview.azure.com",
"scan": "https://tomdevpurview22.scan.purview.azure.com",
"guardian": "https://tomdevpurview22.guardian.purview.azure.com"
},
"provisioningState": "Succeeded",
"privateEndpointConnections": [],
"managedResources": {
"resourceGroup": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/managed-rg-tomdevpurview22",
"storageAccount": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/managed-rg-tomdevpurview22/providers/Microsoft.Storage/storageAccounts/scanwesteuropenfbammd",
"eventHubNamespace": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/managed-rg-tomdevpurview22/providers/Microsoft.EventHub/namespaces/Atlas-c542de27-f8b4-4661-bf57-c17f24058157"
},
"publicNetworkAccess": "Enabled",
"managedResourceGroupName": "managed-rg-tomdevpurview22",
"systemData": {
"createdBy": "redacted@somedomain.com",
"createdByType": "User",
"createdAt": "2021-08-17T10:26:06.8974892Z",
"lastModifiedBy": "redacted@somedomain.com",
"lastModifiedByType": "User",
"lastModifiedAt": "2021-08-17T10:26:06.8974892Z"
}
},
"id": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/tom-devrg/providers/Microsoft.Purview/accounts/tomdevpurview22",
"name": "tomdevpurview22",
"type": "Microsoft.Purview/accounts",
"location": "westeurope",
"identity": {
"type": "SystemAssigned",
"principalId": "00000000-0000-0000-0000-000000000000",
"tenantId": "00000000-0000-0000-0000-000000000000"
},
"tags": {}
}
```

Having checked our test suite, we believe this is a regression which has rolled out within the last 24h:


Would it be possible for someone to take a look?

cc @grayzu