[Feature] - Handle VPC deletion when attached to other resources

[optik-aper]

Describe the bug
VPCs aren't properly deleted when they are attached because there is no way to check what it's attached to. With VPC 2.0 this might be possible so we should support checking if the network is attached to instances before we allow deletion.

To Reproduce

• Create a VPC, instance or load balancer and connect them

resource "vultr_instance" "oof" {
  region = "mel"
  tags = ["terraform"]
  label = "tf-lb-test"
  plan = "vhp-2c-2gb-amd"
  os_id = 1946
}

resource "vultr_vpc" "bar" {
  region   = "mel"
  description = "bar"
}

resource "vultr_load_balancer" "foo" {
  region   = "mel"
  label       = "%s"

  forwarding_rules {
    frontend_protocol = "http"
    frontend_port     = 80
    backend_protocol  = "http"
    backend_port      = 80
  }
  vpc = vultr_vpc.bar.id
  attached_instances = [vultr_instance.oof.id]
}

• Remove the VPC

resource "vultr_instance" "oof" {
  region = "mel"
  tags = ["terraform"]
  label = "tf-lb-test"
  plan = "vhp-2c-2gb-amd"
  os_id = 1946
}

# resource "vultr_vpc" "bar" {
#   region   = "mel"
#   description = "bar"
# }

resource "vultr_load_balancer" "foo" {
  region   = "mel"
  label       = "%s"

  forwarding_rules {
    frontend_protocol = "http"
    frontend_port     = 80
    backend_protocol  = "http"
    backend_port      = 80
  }
  # vpc = vultr_vpc.bar.id
  attached_instances = [vultr_instance.oof.id]
}

• The delete will fail because it's trying to delete the VPC before it is removed/detached from the load balancer

Expected behavior
Correctly determine what is attached to the VPC and remove/detach it before deletion