Require Ruby >= 3.3 and update CI Ruby versions

[dduugg]

Summary

Raises the minimum supported Ruby version to 3.3 (Ruby 3.2 is now EOL) and refreshes CI accordingly.

Changes:

• singed.gemspec: required_ruby_version bumped from >= 3.2.0 to >= 3.3.
• .github/workflows/build.yml: dropped Ruby 3.2 from the CI test matrix; the matrix now runs 3.3, 3.4, and 4.0.
• .rubocop.yml: AllCops.TargetRubyVersion bumped from 3.2 to 3.3.

No .ruby-version / .tool-versions files exist, and .standard.yml has no TargetRubyVersion, so no changes were needed there. The README contains no explicit minimum-Ruby statement to update.

Motivation

Resolves the Ruby version restrictions from rubyatscale/pack_stats#56, where supporting EOL Ruby forced older/vulnerable transitive dependencies. Dropping EOL Ruby 3.2 allows the supported set (3.3 and 3.4) to track current, non-vulnerable 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.