With https://gitlab.com/gitlab-org/gitlab-ce/issues/50289 we moved CI
templates into gitlab repository.
The previous gitlab-ci-yml maintainers are now marked as code owners.
Auto DevOps is assigned to configure team backend developers.
This ports the changes for the CODEOWNERS feature to CE:
- It adds the CODEOWNERS file.
- It adds the mention of the `with-codeowners` branch in TestEnv