* Add Ruby 3.1 to CI
Update Rubocop for recent Rubies
Disable Rubocop run for Rubies before Ruby 2.4
Quote '3.0' in the CI configuration to ensure it loads a 3.0.x Ruby
Set RUBYOPT="--disable_error_highlight" so Ruby 3.1 error matchers pass
* Add CHANGELOG.md entry
* Re-add deleted line from CHANGELOG.md
* Set minimum supported Ruby version to 2.4.
Remove a number of code bits designed to support Rubies below version 2.4
* Bump version. Remove unneeded require from Gemfile. Add require to spec/support file