[Soft Delete] - Soft delete is in private preview

### Describe the bug

Soft delete feature is in private preview and the option for soft delete should be hidden till we are ready for public preview.

### Related command

az sig create --gallery-name
--resource-group
[--description]
[--eula]
[--location]
[--permissions {Community, Groups, Private}]
[--public-name-prefix]
[--publisher-email]
[--publisher-uri]
[--soft-delete {false, true}]
[--tags]

### Errors

Since the feature is in private preview, customers cannot use it and generating support incidents.

### Issue script & Debug output

Since the feature is in private preview, customers cannot use it and generating support incidents.

### Expected behavior

The soft delete feature should be hidden in the command list.

### Environment Summary

Azure CLI Latest

### Additional context

_No response_