{AzurePostgresql} PostgreSQL flexibleServers doesn't support creation…

… of 'SystemAssigned' resource identity (Azure#23423)

Fixes Azure#23176

Per the [latest API](https://github.com/Azure/azure-rest-api-specs/edit/main/specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/stable/2022-12-01/FlexibleServers.json), the flexibleServers should support creation of 'SystemAssigned' , but it doesn't . See the below for details:

Request:
PUT /subscriptions/?/resourceGroups/acctestRG-postgresql-230320155838217513/providers/Microsoft.DBforPostgreSQL/flexibleServers/acctest-fs-230320155838217513?api-version=2022-12-01

```
{"identity":{"type":"SystemAssigned"},"location":"eastus","properties":{"administratorLogin":"adminTerraform","administratorLoginPassword":"QAZwsx123","availabilityZone":"2","backup":{"geoRedundantBackup":"Disabled"},"highAvailability":{"mode":"Disabled"},"network":{},"storage":{"storageSizeGB":32},"version":"12"},"sku":{"name":"Standard_D2s_v3","tier":"GeneralPurpose"},"tags":{}}
```

Response:
```
{"error":{"code":"CannotSetResourceIdentity","message":"Resource type 'Microsoft.DBforPostgreSQL/flexibleServers' does not support creation of 'SystemAssigned' resource identity. The supported types are 'UserAssigned'."}}
```

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/stable/2022-12-01/FlexibleServers.json

@@ -818,7 +818,6 @@
           "description": "the types of identities associated with this resource; currently restricted to 'SystemAssigned and UserAssigned'",
           "enum": [
             "None",
-            "SystemAssigned",
             "UserAssigned"
           ],
           "x-ms-enum": {