Require Ruby >= 3.3 and update CI Ruby versions

[dduugg]

Summary

Ruby 3.2 is now EOL. This PR raises the minimum supported Ruby to 3.3 and refreshes CI so the test matrix targets the supported set (3.3 and 3.4).

Motivation: resolve Ruby version restrictions from rubyatscale/pack_stats#56, where supporting EOL Ruby forced older/vulnerable transitive dependencies.

Changes

• packs-specification.gemspec: required_ruby_version raised from >= 3.1 to >= 3.3.
• .github/workflows/ci.yml: RSpec matrix now tests 3.3, 3.4, and 4.0 (dropped 3.2). The type-check job stays pinned at 3.3 (already supported).
• .rubocop.yml: TargetRubyVersion raised from 2.6 to 3.3.

Notes

• No .ruby-version / .tool-versions / .standard.yml files exist in this repo, and the README has no minimum-version statement to update.
• No bundle install was run and Gemfile.lock is unchanged.
• Latest stable Ruby is 3.4.9; 3.5 is preview-only and not included.

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.