Fix feature fiber storage

[YashaVinter]

Fixes #735.

Summary

i18n 1.15.0 uses Fiber[:i18n_config], but Fiber.[] is only available starting from Ruby 3.2. The README already documents Ruby 3.2+ support, but the gemspec still allowed Ruby 3.1, which lets Bundler install a version that fails at runtime on Ruby 3.1.

This PR updates the gemspec to require Ruby >= 3.2 and adds Ruby 3.2 back to the CI matrix so the minimum supported Ruby version is tested.

Testing

I could not run the full test suite locally because my local Ruby is 3.1.7 and dependencies are not installed.

I verified locally that Ruby 3.1.7 does not support Fiber.[]:

Fiber.respond_to?(:[])
# => false

[radar]

Thanks!

[chadlwilson]

Would it be possible to consider yanking 1.15.0? It now gets automatically installed for anything needing to support Ruby 3.1 (where not locked, which is common for matrix testing scenarios); or suggested to install by dependabot.

That also includes JRuby 9.4 which is only just in soft EOL now.

[Fryguy]

Agree with the above on yanking, or perhaps fix a different way to allow 3.1? I have a number of apps and gems that need to support multiple Rubies, including Ruby 3.1, and with this change, Ruby 3.1 is just broken because 1.15.0 gets installed.