Warn user for using the v1 version.

[Jorropo]

Fixes #1754.5 :

$ go build .
# berty.tech/core/cmd/berty
./warning.go:3:3: warning: #warning This version is outdated please use github.com/berty/berty/v2 instead. [-Wcpp]
    3 | // #warning This version is outdated please use github.com/berty/berty/v2 instead.
      |   ^~~~~~~

It may be placed elsewhere in the repo (I would suggest to move it in the entry point of the lib) so user using go mod would also get the warn but I'm not sure of where it is.