Bump API version to latest (2021-04-01) for Resources/Management Group

### Description

Hello,

I am from the Management Groups dev team and I want to update our PowerShell commands to the latest API version and I would like to know how I can get started. I took a look at the Resource/ManagementGroups and it seems to point to our dotnet sdk which does not look like it was updated for a while: [ManagementGroups Dotnet SDK](https://github.com/Azure/azure-sdk-for-net/tree/main/sdk/managementgroups/Microsoft.Azure.Management.ManagementGroups )

It seems like if the SDK gets updated that should fix the issue but I wanted to make sure. I performed a similar task for our CLI. Thanks!

### Script or Debug output

_No response_

### Environment data

_No response_

### Module versions

_No response_

### Error output

_No response_

Start by reviewing the Resource/ManagementGroups implementation and the linked Microsoft.Azure.Management.ManagementGroups .NET SDK to determine where the API version is selected. Confirm how the PowerShell commands consume that SDK; done means the Resources/Management Group commands target API version 2021-04-01 and their existing validation still passes.

Written by the indexing model from the issue text.