Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Ruby 2.6 offers better performance and memory usage:
https://www.rubyguides.com/2018/11/ruby-2-6-new-features/

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323

This reverts commit 026c92d5, reversing
changes made to 8cda9d33.

The Google GPG key used to sign the apt repository expired today, so all
installs will fail unless `--allow-unauthenticated` is used. We should
revert this once the repository has been updated with the more recent
key.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60441

GitLab is now shipping with Ruby 2.5.3, so we should make the
version consistent for the QA image as well.

jessie doesn't support `ssh-keygen -l -E` but stretch does:
https://manpages.debian.org/jessie/openssh-client/ssh-keygen.1.en.html

[ci skip]