Problem/Motivation
.gitlab-ci.yml in 11.x has additional steps to enable corepack so we can use Yarn 4, but this means future changes to the file will be trickier to backport. However, we can safely enable corepack in 10.3.x and continue to use Yarn 1 there, which keeps the CI config in sync and makes backports simpler.
Steps to reproduce
Proposed resolution
Copy most/all of .gitlab-ci.yml from 11.x to 10.3.x.