This PR drops Ruby 2.6 runtime support. The next release will be the time to drop Ruby 2.6 support:

- https://www.ruby-lang.org/en/downloads/branches/
- https://docs.rubocop.org/rubocop/compatibility.html#support-matrix

CRuby 2.6 compatible JRuby 9.3 will be dropped and CRuby 2.7 compatible JRuby 9.4+ is required.