TUN-9171: Use `is_default_network` instead of `is_default` to create vnet's

## Summary
The is_default field in the request body of the POST /virtual_networks endpoint has been
deprecated and should no longer be used. Clients should use the `is_default_network` field
instead for setting the default virtual network.

Closes TUN-9171

cfapi/virtual_network.go

@@ -16,7 +16,7 @@ import (
 type NewVirtualNetwork struct {
 	Name      string `json:"name"`
 	Comment   string `json:"comment"`
-	IsDefault bool   `json:"is_default"`
+	IsDefault bool   `json:"is_default_network"`
 }
 
 type VirtualNetwork struct {