Shoot with VMO (VMSS Flex) cannot be deleted when the underlying resource group is deleted

[Ismail Alidzhikov]

/area quality
/kind bug
/priority 3
/platform azure

What happened:
Shoot with VMO (VMSS Flex) cannot be deleted when the underlying resource group is deleted

What you expected to happen:
Shoot with VMO (VMSS Flex) to be deleted when the underlying resource group is deleted

How to reproduce it (as minimally and precisely as possible):

1. Create Shoot with VMO (VMSS Flex)

2. Delete manually the underlying resource group

3. Make sure that the Worker deletion fails with:

status:
    lastError:
      codes:
      - ERR_CONFIGURATION_PROBLEM
      description: 'Error deleting worker: failed to cleanup machine dependencies:
        compute.VirtualMachineScaleSetsClient#List: Failure responding to request:
        StatusCode=404 -- Original Error: autorest/azure: Service returned an error.
        Status=404 Code="ResourceGroupNotFound" Message="Resource group ''shoot--foo--bar'
        could not be found."'

Anything else we need to know?:
See also gardener/machine-controller-manager-provider-azure#19

Environment:

• Gardener version (if relevant):
• Extension version: v1.21.2
• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• Others: