4.3.0

• #251: Add SimpleCov and Coveralls reporting in CI and update status badges - @dblock.
• #250: Modernize CI actions and MongoDB startup while retaining Ruby 2.6–4.0, Mongoid 4–9, and MongoDB 4.4–8.0 coverage - @dblock.
• #249: Gate conditional :if/:unless validator support behind a .check_conditions method - @johnnyshields.
• #249: Raise informative error if conditional :if/:unless validator is used with a class subject - @johnnyshields.
• #249: Support Arrays and Procs with non-zero arity for :if/:unless validator - @johnnyshields.
• #248: Add frozen_string_literal: true to all files and enforce rubocop - @johnnyshields.
• #248: Various small CI fixes - @johnnyshields.
• #247: Migrate to danger-pr-comment workflow - @dblock.