Ensure automatic OS upgrades are enabled for Azure Virtual Machine Scale Set

Automatic OS upgrades are disabled for Azure Virtual Machine Scale Set, this may leave it vulnerable to malware and other threats.

In Azure Console -

1. Open the Azure Portal and go to Virtual Machine Scale Sets.
2. Choose the Virtual Machine scale set you wish to edit.
3. Under Settings, select Upgrade policy.
4. Set the Upgrade mode to Automatic or Rolling.

In Terraform -
Deprecated in favor of azurerm_linux_virtual_machine_scale_set and azurerm_windows_virtual_machine_scale_set:

1. In the azurerm_virtual_machine_scale_set resource, set automatic_os_upgrade to true.
2. Set upgrade_policy_mode to either Automatic or Rolling.

References:
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/virtual_machine_scale_set#automatic_os_upgrade