Replace YAML anchors with `extends` in `.gitlab-ci.yml`
Description
In 11.3 we are going to release extends keyword support that is an alternative to YAML anchors.
extends is a little more flexible because it uses reverse, recursive, deep merge strategy, unlike anchors.
We should consider refactoring our own .gitlab-ci.yml to use extends instead of YAML anchors, what should hopefully simplify our CI/CD configuration.