Upgrade CI image to Go 1.12

This is needed for Gitaly, which now requires Go 1.12.

Closes https://gitlab.com/gitlab-org/gitlab/issues/37937

.gitlab-ci.yml

-image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.3-golang-1.11-git-2.22-chrome-73.0-node-12.x-yarn-1.16-postgresql-9.6-graphicsmagick-1.3.33"
+image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.3-golang-1.12-git-2.22-chrome-73.0-node-12.x-yarn-1.16-postgresql-9.6-graphicsmagick-1.3.33"
 
 stages:
   - sync

doc/development/pipelines.md

@@ -42,9 +42,9 @@ The current stages are:
 ## Default image
 
 The default image is currently
-`registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.3-golang-1.11-git-2.22-chrome-73.0-node-12.x-yarn-1.16-postgresql-9.6-graphicsmagick-1.3.33`.
+`registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.3-golang-1.12-git-2.22-chrome-73.0-node-12.x-yarn-1.16-postgresql-9.6-graphicsmagick-1.3.33`.
 
-It includes Ruby 2.6.3, Go 1.11, Git 2.22, Chrome 73, Node 12, Yarn 1.16,
+It includes Ruby 2.6.3, Go 1.12, Git 2.22, Chrome 73, Node 12, Yarn 1.16,
 PostgreSQL 9.6, and Graphics Magick 1.3.33.
 
 The images used in our pipelines are configured in the