bump minimum go version to 1.12

This is one of the changes that was extracted from !274 (merged).

Based on @vshushlin's quote from there:

Based on !160 (merged) and the fact that omnibus is already using 1.13 I'd say that dropping support for 1.11 and 1.12 is fine.

Also, from https://golang.org/doc/devel/release.html :

Each major Go release is supported until there are two newer major releases. For example, Go 1.5 was supported until the Go 1.7 release, and Go 1.6 was supported until the Go 1.8 release. We fix critical problems, including critical security problems, in supported releases as needed by issuing minor revisions (for example, Go 1.6.1, Go 1.6.2, and so on).

So it looks like supporting more than 1 version back is actually a security problem. =)

cc @vshushlin