Upgrade to Ruby 2.6.3

Ruby 2.6.1 should be faster and use less memory than Ruby 2.5.3 (https://meta.discourse.org/t/ruby-2-6-1-will-speed-discourse-up/108281), thanks to a lot of great optimizations mentioned in https://youtu.be/ZfgxvNUfQdU.

Now that Ruby 2.6.1 fixes a show-stopper bug (https://www.ruby-lang.org/en/news/2019/01/30/ruby-2-6-1-released/), let's get our tests passing and be prepared to upgrade.

Note that last time we upgraded from 2.4 to 2.5, we did have some issues having a no-downtime deploy because the 2.4 Gem files were removed before the Unicorn workers were HUP'ed. I think the way to solve this is to take nodes out of the load balancer before installing GitLab.

To do:

• Upgrade gRPC: https://github.com/grpc/grpc/issues/17594
• Upgrade protobuf: https://github.com/protocolbuffers/protobuf/issues/5161
• Upgrade Rubocop 0.68.1: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28046, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11982
• Upgrade Gitaly: gitaly!1228 (merged)
• Upgrade CE/EE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28117, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11535
• Upgrade Omnibus GitLab: omnibus-gitlab!3119 (merged)
• Upgrade Cloud Native GitLab: gitlab-org/build/CNG!224 (diffs)
• Upgrade gitlab-shell: gitlab-shell!298 (merged)

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9449 shows that most of our code is now Ruby 2.6-compliant.