Set the tested Ruby runtime floor

[ryanduguid]

Summary

• replace the untested >= 2.3 Ruby claim with >= 3.2
• align package metadata with the companion CI floor in ci: test the Ruby event source #385

Why

The committed lockfile requires Bundler 2.5.11, whose own Ruby requirement is >= 3.0. The repository had no execution proof for Ruby 2.3, while the companion matrix runs the package on Ruby 3.2. This PR intentionally makes the contract no broader than the tested floor.

Validation

• Ruby 3.2.11 satisfies the new gem requirement; Ruby 3.1.9 does not
• all 560 library/spec Ruby files compile on Ruby 3.2.11
• git diff --check

The PR remains draft until #385 produces exact-head hosted Linux and Windows evidence at Ruby 3.2.

[ryanduguid]

Interaction note from the queue audit: this runtime-floor change shares xero-ruby.gemspec with licence metadata PR #384 and depends on hosted matrix evidence from CI PR #385. Recommended order is #384, then #385, then rebase and rerun this PR against both results. I am keeping it draft until those gates are available.