Upgrade Ruby to 3.4.1 and drop support for EOL versions

[Copilot]

This PR upgrades Ruby to the latest stable version (3.4.1) and drops support for end-of-life Ruby versions to keep the project up-to-date with modern Ruby standards.

Changes Made

• Ruby Version: Updated .ruby-version and .tool-versions to Ruby 3.4.1 (latest stable)
• Minimum Requirements: Raised minimum Ruby version from >= 2.6 to >= 3.0 in gemspec
• CI Matrix: Removed testing for EOL Ruby versions 2.6 and 2.7, now tests Ruby 3.0-3.4
• RuboCop Configuration: Updated TargetRubyVersion to 3.0 to match new minimum
• Code Cleanup: Removed outdated TODO comment about Hash#transform_keys (available since Ruby 2.5)
• Version Bump: Updated gem version to 1.13.0 to reflect the breaking change
• Documentation: Updated CHANGELOG.md with upgrade details

Breaking Change

This is a breaking change as it drops support for Ruby 2.6 and 2.7, which reached end-of-life. Projects using this gem will need to upgrade to Ruby 3.0 or later.

Rationale

• Ruby 2.6 reached EOL in March 2022
• Ruby 2.7 reached EOL in March 2023
• Ruby 3.4 was released in December 2024 with performance improvements and new features
• Dropping EOL versions allows the project to leverage modern Ruby features and reduces maintenance burden

Testing

All existing tests pass with the new Ruby requirements. The gem functionality remains unchanged - this is purely a Ruby version upgrade.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/ruby/ruby/releases/latest
  • Triggering command: curl -s REDACTED (http block)
• https://api.github.com/repos/ruby/ruby/tags
  • Triggering command: curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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