Update to Ruby 2.6.6 for project

Ruby 2.6.6 is the latest stable version: https://www.ruby-lang.org/en/news/2020/03/31/ruby-2-6-6-released/.

Ruby 2.6.6 is set in the .ruby-version for:

• omnibus-gitlab: omnibus-gitlab!4261 (merged)
• gitlab: gitlab!33253 (merged). GitLab has been tested in 2.6.6 for weeks: gitlab!30909 (merged)

I've refreshed the dependencies with 2.6.6 locally also. The site builds and previews fine.

This also updates out Docker image to 2.6.6 also to match.

CC @gl-docsteam.