Use ipv6 CIDR only as ID for IPv6 only subnets. (#1158) (#1163)

Using IPv6 CIDR in ID string will break the migration of subnets from
IPv4 to dual-stack. Due to the different ID subnets would be deleted and
crated again. This already breaks the migration of IPv4 shoots to shoots
wiht dual-stack ingress.

Co-authored-by: Axel Siebenborn <axel.siebenborn@sap.com>

pkg/controller/infrastructure/infraflow/reconcile.go

@@ -725,7 +725,7 @@ func (c *FlowContext) ensureZones(ctx context.Context) error {
 		return err
 	}
 	toBeDeleted, toBeCreated, toBeChecked := diffByID(desired, current, func(item *awsclient.Subnet) string {
-		if item.Ipv6CidrBlocks != nil {
+		if item.Ipv6CidrBlocks != nil && item.CidrBlock == "" {
 			return item.AvailabilityZone + "-" + item.Ipv6CidrBlocks[0]
 		}
 		return item.AvailabilityZone + "-" + item.CidrBlock