Support apollo client 4

[rmosolgo]

Fixes #5661

This is a pretty big switch so I think it require a version bump in graphql-ruby-client, too.

Major changes:

• Update imports
• Use Observable everywhere
• Apollo now calls the link-provided unsubscribe function when observable is completed, so I removed some spots where the Link unsubscribed from the transport layer (it was producing double-unsubscribes in tests).