As a follow-up to #37953, we can improve the atLeastGo1 and atMostGo1 helpers so that they operate closer to what the name suggests, and report only whether the Go version meets a certain number. Right now they report suboptimal results on non-release branches in some situations.

For example, the master branch is currently where Go 1.15 is being developed, but atLeastGo1(goBranch, 16) reports true. It should be false.

Similarly, atMostGo1(goBranch, 15) makes an incorrect assumption that master is already greater than 1.15, and reports false. It should be true.

cmd/coordinator can look up precise Go release information (via maintnerClient), and use it to make a better decision. We'll need to modify the signature of BuildsRepoPostSubmit to pass more information to it.

/cc @toothrot @cagedmantis