Ask for PG11 with GDK
Because of: https://about.gitlab.com/releases/2020/02/22/gitlab-12-8-released/#postgresql-11-is-now-available-in-omnibus-gitlab, and having successfully used PG11 with GDK + Geo, it's time for us to have all GDK uses tending toward PG11.
As a macOS user, I've only updated instructions I know will work for macOS. I've left the work of updating for other platforms for future contributors.