Bump minimum Ruby version to 3.1

[tvdeyen]

Summary

Extracted from #5843 for better visibility

We need at least Ruby 3.1 for Rails 7.2 compatibility and Ruby 3.1 is EOL anyway.

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 87.94326% with 17 lines in your changes missing coverage. Please review.

Project coverage is 89.58%. Comparing base (5a9dc62) to head (a9c88c9).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
core/lib/spree/core/importer/product.rb	0.00%	3 Missing ⚠️
...dels/concerns/spree/ordered_property_value_list.rb	0.00%	2 Missing ⚠️
.../lib/spree/testing_support/dummy_app/rake_tasks.rb	0.00%	2 Missing ⚠️
...ore/lib/spree/testing_support/order_walkthrough.rb	0.00%	2 Missing ⚠️
...controllers/spree/admin/option_types_controller.rb	0.00%	1 Missing ⚠️
core/app/mailers/spree/base_mailer.rb	0.00%	1 Missing ⚠️
core/app/mailers/spree/order_mailer.rb	66.66%	1 Missing ⚠️
core/app/models/spree/payment_create.rb	0.00%	1 Missing ⚠️
core/app/models/spree/reimbursement.rb	66.66%	1 Missing ⚠️
core/lib/spree/mailer_previews/carton_preview.rb	0.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5891      +/-   ##
==========================================
+ Coverage   87.88%   89.58%   +1.70%     
==========================================
  Files         477      783     +306     
  Lines       11659    17991    +6332     
==========================================
+ Hits        10246    16117    +5871     
- Misses       1413     1874     +461     

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[nvandoorn]

Thanks @tvdeyen! Been talking about this change with other folks at Super Good.