Drop runtime support for Ruby 3.2

[Earlopain]

👋 For #348, I would like to replace RubyVM::AbstractSyntaxTree with the prism gem. However prism only knows how to parse Ruby 3.3+ code.

Ruby 3.2 will be EOL by end of march this year. Is it ok for you to drop ruby 3.2 support when that happens? Recently you seem to have dropped EOL rubies relatively quickly. Or, perhaps for rbs 4.0 this is ok to do even now? (I don't know if you have release plans)

Thanks for considering (:

cc @eregon