[Feature]: Add warning for users of eventual change of default for "az vmss create"

[thisisanniefang]

az vmss create currently creates a Basic LB by default, we will eventually want to make the default to create a Standard LB instead.

In advance of eventual breaking change, would like to add warning for users to encourage them to use --lb-sku parameter with Standard.

Similar change done for "az vm create", "az network lb create", and "az network lb frontend-ip create"- #21831, #18662

[yonzhan]

Compute