go: migrate from Gopkg to modules

[JulienBalestra]

I kept the dep at the version they were in the first place, to ease the review:

go mod init
go: creating new go.mod: module github.com/uswitch/nidhogg
go: copying requirements from Gopkg.lock

[Joseph-Irving]

So I tried doing this myself, but I couldn't get it to work as kubebuilder v1 doesn't support go mod, the v2 does but it's a bit of a pain to migrate to it https://book.kubebuilder.io/migration/guide.html