Update .ruby-version to 2.6.6
What does this MR do?
Updates .ruby-version
to 2.6.6, to match latest stable version: https://www.ruby-lang.org/en/news/2020/03/31/ruby-2-6-6-released/.
We've been running 2.6.6 in CI pipelines for a few weeks now: !30909 (merged).
Will require contributors to upgrade to Ruby 2.6.6.
Edited by Evan Read