Rename model dcim.ConnectedEndpoint to dcim.ConnectedEndpoints and it conform to the new cable termination model from 3.3.x

[ChilloManiac]

In netbox 3.3.x cables support multiple terminations on either end - The corresponding model of e.g. an interface uses connected_endpoints now instead of connected_endpoint and contains a list of interfaces.

Pynetbox version: 6.6.2