This ticket should update the places where we check a client's wire protocol version to

• decide what maxWireVersion to send back in the isMaster handshake (to make the client fail to connect if it's too old)
• decide whether to close client connections from clients that are too old
• decide whether a shard is too old to add to a cluster

particularly to check "in any state other than fully downgraded to 3.4" rather than just checking "in 3.6."