Upgrade to Ruby 3.4

[jvendetti]

Upgrade this project from Ruby 3.2 to 3.4 (resolves #53).

• Require Ruby 3.4 in the gemspec
• Track .ruby-version in the repository
• Update CI to test Ruby 3.4

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.41%. Comparing base (8549896) to head (04eded6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   66.48%   66.41%   -0.08%     
==========================================
  Files          29       29              
  Lines         919      920       +1     
  Branches      174      175       +1     
==========================================
  Hits          611      611              
- Misses        308      309       +1     

Flag	Coverage Δ
3.2	66.48% <ø> (ø)
3.4	66.44% <ø> (?)

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