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; the supported set is 3.3 and 3.4).

Changes

• parse_packwerk.gemspec: spec.required_ruby_version raised from '>= 2.6' to '>= 3.3'.
• .rubocop.yml: AllCops.TargetRubyVersion raised from 2.6 to 3.3.

Notes

• No .ruby-version / .tool-versions files exist in this repo, so none were changed.
• CI workflows (.github/workflows/*.yml) delegate to rubyatscale/shared-config reusable workflows and contain no inline Ruby version matrices, so no workflow edits were needed.
• The README has no explicit minimum-Ruby-version statement to update.
• Remaining 2.7/3.0/3.1 references are confined to generated Sorbet RBI files and version-specific comments, which were intentionally left untouched.

Motivation

Resolves the Ruby version restrictions from rubyatscale/pack_stats#56, where supporting EOL Ruby forced older/vulnerable transitive dependencies.

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.