AKS - change default instance type since microsoft is removing it

[mantis-toboggan-md]

The current default instance type when provisioning AKS through the UI is Standard_DS2_v2

dsv2 instance types will be deprecated - https://learn.microsoft.com/en-us/azure/virtual-machines/sizes/general-purpose/dv2-series?tabs=sizebasic

If the UI can't find the default instance type in the API request to fetch available instance types, it will fall back to the first listed alphabetically. Effectively, when Standard_DS2_v2 goes away in May, our UI will NOT break, but our new default AKS instance type will be "whatever shows up first in the list"

We should check if the typical first entry is an appropriately-sized general purpose instance type, or if there is a more reasonable value to default to - instance types have an enormous impact on the overall cost of running the cluster, so it is important that our default value is sensible.

[izaac]

@IsaSih are these defaults covered in the E2E AKS tests already? If so we can move this to done and take a look to see if the tests need some updates (these doesn't run in GH actions)

[IsaSih]

The tests would cover the old default values. I'll update issue https://github.com/rancher/qa-tasks/issues/1570

[IsaSih]

Works as expected. UI is displaying the correct value for VM size and AKS is being successfully provisioned with the default value.

Validated in Rancher v2.12-0d5e9dc2323c35d68fdaffbb7a1907f2b844a4b8-head Dashboard master 8dd0141 Rancher CLI v2.12.0-rc.1