Bump ruby version, gem versions and some code fixes to improve overal…

[abhig10]

…l state

Summary

This PR modernizes rubocop-changes for current Ruby versions and hardens command/output handling.

• Raises minimum supported Ruby to >= 3.3 and aligns tooling (Gemfile, gemspec, .ruby-version, CI matrix, RuboCop target version).
• Updates dependencies to current compatible versions (including RSpec/Rake updates in Gemfile.lock).
• Improves command safety in Checker by shell-escaping dynamic values (base branch, commit, fork point, changed file paths) before building git/rubocop commands.
• Fixes a runtime crash from non-ASCII shell output (invalid byte sequence in US-ASCII) by sanitizing command output to UTF-8 in Shell.run.
• Adds regression coverage for shell output encoding handling (spec/rubocop/changes/shell_spec.rb) and updates specs/config/docs for consistency and lint cleanliness.
• Minor maintenance updates: modernized checkout action in CI and small cleanup/typo/style fixes across code and README.