SecurityInsights API: Alert Rule Threat Intelligence defines "severity" as read-only

In the [security insight alert rule API spec](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/securityinsights/resource-manager/Microsoft.SecurityInsights/preview/2019-01-01-preview/SecurityInsights.json#L4424), it defines the "severity" property as a read-only property. However, the backend service requires the `severity` to be set, otherwise, it returns:

```json
{
"error": {
"code": "BadRequest",
"message": "Required property 'severity' not found in JSON. Path 'properties', line 3, position 17."
}
}
```

This is a mismatch between the swagger spec and the service behavior. Needs either to change the spec or change the backend service implementation.

Open specification/securityinsights/resource-manager/Microsoft.SecurityInsights/preview/2019-01-01-preview/SecurityInsights.json at line 4424 and inspect the severity definition and nearby alert-rule schema. Compare that contract with the reported create-request error; done means the published specification and backend agree on whether severity is required and writable.

Written by the indexing model from the issue text.