Issue summary:
You are migrating from classic WVD to AVD and looking for the best way to achieve it. You are also looking for information on the management API.
Recommendation :
The API referenced in your post is the legacy API for WVD classic. It can’t be used for the current ARM-based version of AVD.
The App Group related API is documented here: https://learn.microsoft.com/en-us/rest/api/desktopvirtualization/application-groups
We also offer PowerShell modules to automate the migration from classic to the current model: https://learn.microsoft.com/en-us/azure/virtual-desktop/automatic-migration
Hope that helps.