Consider updating default DEP version to 7 from 3

[jessepeterson]

Latest version of DEP protocol is v7 as noted in Apple docs — example 1. While users of NanoDEP can explicitly set any protocol version they like we set default to 3 here:

nanodep/client/transport.go

Line 19 in 3b6c000

DefaultServerProtocolVersion = "3"

Consider upping the default to the current version 7.