Drop support for Ruby 3.2 and remove dead version guards

[mattmenefee]

Summary

• (Breaking) Drops support for Ruby 3.2, which reached end-of-life on 2026-04-01 and no longer receives security patches.
• Raises the floor to Ruby 3.3 (currently in security maintenance, EOL expected 2027-03-31):
  • chewy.gemspec: required_ruby_version >= 3.2 → >= 3.3
  • .github/workflows/ruby.yml: drop '3.2' from the test matrix (3.3 was already covered)
  • .rubocop.yml: TargetRubyVersion 3.2 → 3.3
  • README compatibility statement and CONTRIBUTING prerequisites
  • Troubleshooting guide: drop the parser / prism version conditional from the Witchcraft optional-dependency list, leaving method_source and unparser (Prism is bundled with Ruby 3.3+ and parser is pulled in transitively by unparser, so neither needs adding to a Gemfile)

Dead version-guard cleanup (exposed by the floor bump)

• Chewy::Index::Witchcraft branched on RUBY_VERSION >= '3.3' to choose between Prism (a default gem since Ruby 3.3) and the external parser gem. The parser fallback in the require block, check_requirements!, and ast_from_proc is now unreachable, so witchcraft always uses Prism — also removing a fragile string version comparison. The Prism requirements check itself is dropped: Prism ships with Ruby 3.3+, so it's always present and never needs adding to a Gemfile; the real parser dependency is already covered transitively by the unparser check.
• Chewy::LogSubscriber branched on ActiveSupport.version >= 7.1, unconditionally true since the AS >= 7.2 floor; the pre-7.1 positional color call is removed.

Version-anchored docs describing past releases' requirements (e.g. the Chewy 7 → 8 migration guide) are intentionally left unchanged.

Test plan

• bundle exec rubocop — no offenses with TargetRubyVersion: 3.3
• gem build chewy.gemspec succeeds with required_ruby_version >= 3.3
• CI green on the Ruby 3.3 / 3.4 / 4.0 matrix

[AlfonsoUceda]

Thanks @mattmenefee I'm planning this for v9