CI: Stop supporting Ruby 3.1, use Ruby 3.2+ in the gemspec

This Issue is a **proposal** to use only in-support Ruby versions for Gemstash.

Ruby 3.1: Ended 4 months and 2 weeks ago (31 Mar 2025)

We don't currently have the generally-available Ruby 3.4 in the build matrix.

- Drop 3.1
- Add 3.4
- Use latest JRuby rather than 9.4 (just have latest JRuby rather than several versions)