CI: Add Ruby 3.4. Drop Ruby 3.0, Ruby 3.1, from test matrix.

[olleolleolle]

This PR updates the test matrix, to include Ruby versions that are current and which the ri18n gem supports.

Relevant snippet from #76.

Because every version of r18n-core depends on Ruby >= 3.2, < 3.5
  and Gemfile depends on r18n-core >= 0,
  Ruby >= 3.2, < 3.5 is required.

Fixes #76

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (cb6e8f5) to head (77764f3).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files           3        3           
  Lines         115      115           
=======================================
  Hits          113      113           
  Misses          2        2           

Flag	Coverage Δ
	?

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[AlexWayfer]

Thank you for the PR.

Sorry for a late response, I missed a notification somehow.

The new version 6.0.0 does the same, and some more.

I hope you'll enjoy.