Move package to go modules

[timohahaa]

Basically this package is very old (but still very useful!), and it doesn't use go-modules (a standard way that go manages dependencies nowadays). This can cause other go-programs, that use this package, to fail on compile time:

make build
GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -ldflags "-s -w" -o .build/api ./cmd/api
# github.com/djimenez/iconv-go
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/reader.go:10:21: undefined: Converter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/reader.go:18:20: undefined: NewConverter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/reader.go:28:58: undefined: Converter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/writer.go:7:21: undefined: Converter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/writer.go:15:20: undefined: NewConverter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/writer.go:25:63: undefined: Converter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/iconv.go:11:20: undefined: NewConverter
../../../go/pkg/mod/github.com/djimenez/iconv-go@v0.0.0-20160305225143-8960e66bd3da/iconv.go:37:20: undefined: NewConverter
make: *** [Makefile:17: build] Error 1

The error persists even if using vendor directory. I just added a go.mod file and fixed go version at 1.24.0 (currently the most up-to-date one).