We don't need Ruby 2.7.4 cluttering our GDK when everything else has
been upgraded to 2.7.5.

Part of https://gitlab.com/groups/gitlab-org/-/epics/2380

GitLab has now been upgraded to use Ruby 2.6.6:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33253

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

Restores some of the changes in
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/257, but
still targets Rubocop for 2.3.

This reverts merge request !257