Backend Go module does not follow Go modules versioning

[invidian]

Latest release of backend module right now is https://github.com/kinvolk/nebraska/releases/tag/2.4.1, however https://github.com/kinvolk/nebraska/blob/main/backend/go.mod#L1 is still pointing to v0/v1 version.

Perhaps v2 should be added there and for all internal imports.

See also https://go.dev/blog/v2-go-modules