mongo gem has dropped Ruby 2.4 support since v2.17.0.
https://www.mongodb.com/docs/ruby-driver/current/reference/driver-compatibility/

bson gem has also dropped it since v4.13.0.
https://github.com/mongodb/bson-ruby/releases/tag/v4.13.0

So Ruby 2.4 limits their versions as follows:

• mongo < 2.17.0
• bson < 4.13.0

It can make our future development difficult.
Actually, it conflicts with JRuby 9.3 which is compatible Ruby 2.6 and requires bson >= 4.14.0.