[BUG] [sentinel@2022-10-01-preview] the `FusionAlertRuleTemplateProperties.Description` `.lastModifiedUtc` , `severity` and `.DisplayName` are read-only but not specified

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/ff1e3a1d1aa5f6a959bf4a214e6f69c4d1057647/specification/securityinsights/resource-manager/Microsoft.SecurityInsights/preview/2022-10-01-preview/AlertRules.json#L1213

### API Spec version

2022-10-01-preview

### Describe the bug

The property is read-only on the service side, try to contains it in payload will result in
```
unexpected status 400 (400 Bad Request) with error: BadRequest: Read-only property 'description' cannot be assigned. Path 'properties.description'
```
However it was not specified in Swagger.

### Expected behavior

It should have a "readonly:true" specified

### Actual behavior

but it does not

### Reproduction Steps

call the API with "description" specified.

### Environment

_No response_