Require Ruby >= 3.3 and update CI Ruby versions

[dduugg]

Summary

Bumps the minimum supported Ruby version to 3.3 (Ruby 3.2 is now EOL) and refreshes the CI test matrices to the currently supported set (3.3 and 3.4).

Changes

• visualize_packs.gemspec: required_ruby_version raised from '>= 2.7' to '>= 3.3'.
• .github/workflows/ci.yml: In both the rspec and visual_regression_suite job matrices, removed Ruby 3.1 and 3.2 and added 3.4, leaving the supported set [3.3, 3.4]. The single-pinned static_type_check step (already '3.3') was left unchanged.
• README.md: Updated the contributing minimum-version statement from "Ruby 3.2.2" to "Ruby 3.3".

Motivation

This resolves the Ruby version restrictions from rubyatscale/pack_stats#56, where continuing to support EOL Ruby versions forced the use of older/vulnerable transitive dependencies. Dropping EOL Ruby (3.2) and standardizing on the supported 3.3/3.4 set unblocks newer dependency versions.

Bundler

Also bumps BUNDLED WITH in Gemfile.lock to bundler 4.0.15 (latest). The previously pinned bundler version crashes on Ruby head (NameError: uninitialized constant Pathname::SEPARATOR_PAT) — see this failing run. Pinning the current bundler resolves it.