The 2.10.3 driver is advertised as still supporting JRuby 1.7 (Ruby 1.9+). However, the 10.3 gemspec requires bson version 4.6, which itself requires Ruby 2.3+/JRuby 9.2+, and therefore it transitively forces 10.3 to become required on Ruby 2.3+/JRuby 9.2+ as well.
The prior version of bson, 4.5, appears to still support older version of Ruby/JRuby.
I see that the upgrading from bson 4.4.2 to bson 4.6 was intended to fix https://jira.mongodb.org/browse/RUBY-1980 in https://github.com/mongodb/mongo-ruby-driver/commit/ffc8ca848f878176786fdf1fe1b478c151ef52d2
Reading the changes from bson 4.5 to bson 4.6: https://github.com/mongodb/bson-ruby/releases/tag/v4.6.0 It is not clear to me if the bson 4.6 changes are needed to support the fix on https://jira.mongodb.org/browse/RUBY-1980. If not, I recommend downgrade to bson 4.5 to restore support for older Ruby.