[BUG] Microsoft.Sql - jobAgents - `JobAgentIdentityType` lists unsupported types in enum

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/sql/resource-manager/Microsoft.Sql/preview/2023-08-01-preview/JobAgents.json#L345-L350

### API Spec version

2023-08-01-preview

### Describe the bug

Based on the enum in the REST API specs it would seem that `SystemAssigned` and `SystemAssignedUserAssigned` are supported identity types, however when sending `SystemAssigned` in the request the API responds with a 400 status code.

Docs: https://learn.microsoft.com/en-us/rest/api/sql/job-agents/create-or-update?view=rest-sql-2023-08-01-preview&tabs=HTTP#jobagentidentitytype

Additionally, if `SystemAssignedUserAssigned` is to be supported, it would be great if the value was updated to `SystemAssigned, UserAssigned` to be consistent with other identity type enums.

### Expected behavior

REST API spec and documentation accurately lists supported types / service accepts all listed types.

### Actual behavior

400 Bad request error

```
performing CreateOrUpdate: unexpected status 400 (400 Bad Request) with error: CannotSetResourceIdentity: Resource type 'Microsoft.Sql/servers/jobAgents' does not support creation of 'SystemAssigned' resource identity. The supported types are 'UserAssigned'.
```

### Reproduction Steps

```
PUT https://management.azure.com/subscriptions//resourceGroups//providers/Microsoft.Sql/servers//jobAgents/?api-version=2023-08-01-preview

{
"identity": {
"type": "SystemAssigned"
},
"location": "eastus",
"name": "",
"properties": {
"databaseId": "/subscriptions//resourceGroups//providers/Microsoft.Sql/servers//databases/"
},
"sku": {
"name": "JA100"
},
}
```

### Environment

_No response_

Start with JobAgents.json in the 2023-08-01-preview Microsoft.Sql specification, especially lines 345-350, and compare the JobAgentIdentityType enum with the linked REST API documentation. Reproduce the PUT request using SystemAssigned and verify that the enum and documentation accurately reflect the service-supported identity types; completion requires resolving the mismatch between the spec and the 400 response.

Written by the indexing model from the issue text.