Meta: Replace Code Climate with golangci-lint

Context from Slack thread:

Pedro:

Question: why do we use Code Climate vs something like golangci-lint?

Steve:

Because of dogfood, code quality only supported Code Climate. But now golandci-lint supports the GitLab output, we also recommend it in https://docs.gitlab.com/ee/development/go_guide/index.html#automatic-linting. So in short: time 🙂