[BUG] GET Microsoft.DBforPostgreSQL/flexibleServers/migrations @ 2023-12-01-preview returns 400

### API Spec link

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2023-12-01-preview/Migrations.json

### API Spec version

2023-12-01-preview

### Describe the bug

I get an error when sending a GET HTTP request to Microsoft.DBforPostgreSQL/flexibleServers/migrations REST API

### Expected behavior

Returns
```
{
"message": null,
"name": "testmigration",
"nameAvailable": true,
"reason": null,
"type": "Microsoft.DBforPostgreSQL/flexibleServers/migrations"
}
```

### Actual behavior

Returns
```
{'error': {'code': 'InvalidInput', 'message': "Invalid input. Parameter name: 'MigrationResourceId'. Error: Invalid migration resource ID 'subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/testResourceGroup/providers/Microsoft.DBforPostgreSQL/flexibleServers/testServer/migrations/testMigration'.\r\nParameter name: MigrationResourceId"}}
```

### Reproduction Steps

Send a GET HTTP request to `https://management.azure.com/subscriptions/${subscription_id}/resourceGroups/${resource_group_name}/providers/Microsoft.DBforPostgreSQL/flexibleServers/${server_name}/migrations/${migration_name}?api-version={api_version}`

### Environment

_No response_