CC-2062: Update Ruby to 3.4

[andy1li]

CC-2062: Update Ruby to 3.4

---

Note

Updates Ruby to 3.4 and replaces app/server.rb with app/main.rb across starters, solutions, and templates; adds a Ruby 3.4 Dockerfile.

• Runtime/Build:
  • Update buildpack and requirements to ruby-3.4 in codecrafters.yml and starter_templates/ruby/config.yml.
  • Add dockerfiles/ruby-3.4.Dockerfile with Alpine base and dependencies; enable bundle install with cache-friendly copy.
• Entrypoint Change:
  • Replace exec bundle exec ruby app/server.rb with ... app/main.rb in .codecrafters/run.sh and your_program.sh across compiled starters, solutions, and templates.
  • Add app/main.rb in starters and solutions; uncomment minimal TCP server in solution.
• Docs:
  • Update READMEs to reference app/main.rb and Ruby 3.4.

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[linear]

CC-2062 Update Ruby to 3.4