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

Attempt to update google-protobuf for migration-paths

Because the one we were using aren't compatible with
Ruby 2.5.3, and it'll be troublesome to switch Ruby.
Upgrading google-protobuf will be much easier.

All of them will need to be updated for Ruby 2.5.3

We remove oj because we don't really need it and it
doesn't compile on 2.5.3 with that version.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41825

Fixes that make this work:

* A change in Ruby (https://github.com/ruby/ruby/commit/ce635262f53b760284d56bb1027baebaaec175d1)
requires passing in the exact required length for OpenSSL keys and IVs.

* Ensure the secrets.yml is generated before any prepended modules are
loaded. This is done by renaming the `secret_token.rb` initializer to
`01_secret_token.rb`, which is a bit ugly but involves the least impact on
other files.

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

Part of gitlab-org/gitlab-ce#38040

Ruby 2.1.8 is used only on master for now, as this would give
confidance in case this commit has to be reverted.

https://www.ruby-lang.org/en/news/2015/04/13/ruby-2-1-6-released/