[BUG] PowerBI Embedded API always returns "Gen2" for the mode property even if the mode property is set to "Gen1" in the request payload

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/powerbidedicated/resource-manager/Microsoft.PowerBIdedicated/stable/2021-01-01/powerbidedicated.json

### API Spec version

2021-01-01

### Describe the bug

PowerBI Embedded API always returns "Gen2" for the mode property even if the mode property is set to "Gen1" in the request payload.

Request Payload:
```
PUT https://management.azure.com/subscriptions/xx-xx-xx-xx-xx/resourceGroups/acctestRG-powerbi-240717125400411087/providers/Microsoft.PowerBIDedicated/capacities/acctestpowerbi240717125400411087

{
"location": "westeurope",
"properties": {
"administration": {
"members": [
"xx-xxx-xxx-xxx"
]
},
"mode": "Gen1"
},
"sku": {
"name": "A1"
},
"tags": {}
}
```

### Expected behavior

PowerBI Embedded API should return "Gen1" for the mode property when the mode property is set to "Gen1" in the request payload.

### Actual behavior

PowerBI Embedded API should return "Gen2" for the mode property when the mode property is set to "Gen1" in the request payload.

### Reproduction Steps

1. Create Resource Group
2. Create PowerBI Embedded with mode = "Gen1"

### Environment

PROD