Dpds_v6 erroneously reporting no support for Ephemeral OS disks

[richshadman]

According to the documentation for the Dpds_v6 family, Ephemeral OS disks are supported: https://learn.microsoft.com/en-us/azure/virtual-machines/sizes/general-purpose/dpdsv6-series?tabs=sizebasic

However when trying to create a VMSS with this family we get an error:

This issue was previously reported in AKS (similar error when provisioning node pool (VMSS backed) with this size): Azure/AKS#4676

For now we went with a lower generation (v5) which does not throw the same error, however we would love to be able to leverage the newer architecture with Ephemeral OS disk (using it as a devops build agent for ARM64 oci images so performance is paramount).