Fix Linode interfaces property

[zliang-akamai]

📝 Description

1. Rename interfaces property of a Linode instance object to linode_interfaces to avoid confusion with legacy config interfaces.
2. Avoid API error by returning None from the property getter function when the Linode is with legacy interfaces.

✔️ How to Test

import os
from linode_api4 import LinodeClient, Instance
from linode_api4.objects.linode_interfaces import LinodeInterfacePublicOptions


linode_client = LinodeClient(
    os.getenv("LINODE_TOKEN"),
    base_url="https://api.linode.com/v4beta",
)

created_linode = linode_client.linode.instance_create(
    ltype="g6-nanode-1",
    region="us-mia",
    interface_generation="legacy_config"
)

linode = linode_client.load(Instance, created_linode.id)
print(linode.linode_interfaces) # expecting None
linode.delete()

created_linode2 = linode_client.linode.instance_create(
    ltype="g6-nanode-1",
    region="us-mia",
    interface_generation="linode"
)
created_linode2.interface_create(
    firewall=12345, # replace with your non-prod firewall here
    public=LinodeInterfacePublicOptions(),
)

linode2 = linode_client.load(Instance, created_linode2.id)
print(linode2.linode_interfaces) # expecting a list with one LinodeInterface object
linode2.delete()

[lgarber-akamai]

LGTM