Merge branch 'sh-update-gce-default-image' into 'main'

gce: Update Ubuntu version to 20.04

See merge request https://gitlab.com/gitlab-org/ci-cd/docker-machine/-/merge_requests/125



Merged-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Approved-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Co-authored-by: Stan Hu <stanhu@gmail.com>