fix ruby version support

[ccarruitero]

Currently gem don't support ruby 2. Wrongly, when added ruby 3 support, ruby was locked into 3 versions
35fbc5c#diff-a63166717e2d725ff120dccad29b7c1cb14b2a3aec6c0d6f8fd28acffe2610d2R17

We have to update gemspec into something like

spec.required_ruby_version = Gem::Requirement.new('> 2.7')