Require MFA for gem pushes

This adds the `rubygems_mfa_required` metadata to the gemspec,
requiring multi-factor authentication for privileged operations
on RubyGems.org.

This is a protection against supply chain attacks like the recent
NPM Axios compromise: https://socket.dev/blog/axios-npm-package-compromised

Reference: https://guides.rubygems.org/mfa-requirement-opt-in/