chore: replace deprecated ioutil with io/os

[galargh]

ioutil is deprecated as of Go 1.16, see https://pkg.go.dev/io/ioutil

This PR replaces usage of ioutil with io or os, accordingly.

This is required to enable Unified CI/Go version upgrade, see #56

I generate examples/gen-routing-api/proto/proto_edelweiss.go with:

cd examples/gen-routing-api
rm -rf proto
go build
./gen-routing-api

I generated examples/greeting-service/api/proto/proto_edelweiss.go with:

cd examples/greeting-service/api
rm -rf proto
go build
./api