Update Ruby to 3.4

[andy1li]

Note

Upgrades Ruby to 3.4 across starters/solutions and adds a Ruby 3.4 Dockerfile with required Alpine dependencies.

• Runtime/version bump:
  • Update required Ruby version to 3.4 in compiled_starters/ruby/README.md, solutions/ruby/*/code/README.md, and starter_templates/ruby/config.yml.
  • Switch buildpack from ruby-3.3 to ruby-3.4 in compiled_starters/ruby/codecrafters.yml and solutions/ruby/*/code/codecrafters.yml (and update available versions comment).
• Docker:
  • Add dockerfiles/ruby-3.4.Dockerfile using ruby:3.4-alpine3.23, installs necessary packages, sets dependency file paths, copies app, and runs bundle install.

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