Remove support for Go 1.9
Go 1.9 is no longer supported by the Go team. I would like to drop it from the CI matrix in Gitaly, but that opens the door to Gitaly becoming incompatible with Go 1.9. So we'd have to say that GitLab as a whole requires Go 1.10 or newer.
I don't want to break the omnibus build with this. Is Go 1.10 an acceptable minimum version for everything in the build? Or do we support having multiple Go compilers at build time?