CC-2062: Update Ruby to 3.4

[andy1li]

CC-2062: Update Ruby to 3.4

---

Note

Updates Ruby runtime to 3.4 across docs/config and adds a new ruby-3.4 Dockerfile.

• Ruby version bump to 3.4:
  • Update requirements in compiled_starters/ruby/README.md and solutions/ruby/*/code/README.md to ruby (3.4).
  • Switch buildpack in compiled_starters/ruby/codecrafters.yml and solutions/ruby/*/code/codecrafters.yml to ruby-3.4.
  • Update starter_templates/ruby/config.yml required_executable to ruby (3.4).
• Docker:
  • Add dockerfiles/ruby-3.4.Dockerfile using ruby:3.4-alpine3.23, installs build/runtime deps, sets CODECRAFTERS_DEPENDENCY_FILE_PATHS, copies app, and runs bundle install.

^{Written by Cursor Bugbot for commit 506f336. This will update automatically on new commits. Configure here.}

[linear]

CC-2062 Update Ruby to 3.4