Completely update Ruby to 2.4.4
Some ruby version specifications are still not at 2.4 or 2.4.4 after !2488 (merged).
This MR includes removing 2.3 and enforcing them to 2.4.4 or 2.4.
The Performance/RegexpMatch
Cop newly introduced in rubocop-hq/rubocop#3824 for Ruby 2.4+ is set to false
in this MR.
cf.
Closes #3572 (closed)
Edited by GitLab Release Tools Bot