Stop referencing actual CI image and point to .gitlab-ci.yml instead

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/pipelines.md

@@ -38,11 +38,9 @@ The current stages are:
 
 ## Default image
 
-The default image is currently
-`registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.5-golang-1.12-git-2.24-lfs-2.9-chrome-73.0-node-12.x-yarn-1.21-postgresql-9.6-graphicsmagick-1.3.34`.
+The default image is defined in <https://gitlab.com/gitlab-org/gitlab/blob/master/.gitlab-ci.yml>.
 
-It includes Ruby 2.6.5, Go 1.12, Git 2.24, Git LFS 2.9, Chrome 73, Node 12, Yarn 1.21,
-PostgreSQL 9.6, and Graphics Magick 1.3.34.
+It includes Ruby, Go, Git, Git LFS, Chrome, Node, Yarn, PostgreSQL, and Graphics Magick.
 
 The images used in our pipelines are configured in the
 [`gitlab-org/gitlab-build-images`](https://gitlab.com/gitlab-org/gitlab-build-images)