Use StandardV2 NAT Gateways

[doshitan]

Context

See TODO:

template-infra-azure/infra/modules/azure/network/nat_gateway/main.tf

Lines 22 to 28 in 31f21e9

	# TODO: at the moment we create a single NAT Gateway for the Virtual Network, in
	# AWS we create a different one for each availability zone. Do we want to do
	# that here? We'll have to create unique subnets for each zone though, and put
	# zonal resources into the right subnet.
	#
	# https://learn.microsoft.com/en-us/azure/nat-gateway/nat-overview#availability-zones
	# https://learn.microsoft.com/en-us/azure/nat-gateway/nat-availability-zones

The StandardV2 SKU is automatically zone redundant and supports IPv6 (as well as being higher performance), for the same cost: https://learn.microsoft.com/en-us/azure/nat-gateway/nat-sku

Support added in azurerm provider v4.67.0: https://github.com/hashicorp/terraform-provider-azurerm/releases/tag/v4.67.0

Implementation

• Swap the SKU values
  • The StandardV2 NAT also requires using StandardV2 public IP addresses/prefixes
• Provide migration docs
  • https://learn.microsoft.com/en-us/azure/nat-gateway/nat-gateway-v2-migrate

Acceptance Criteria

• NAT Gateways use StandardV2 SKUs by default