feat: upgrade to Rails 8.1

[G-Rath]

This is focused on just having us generate apps with Rails 8.1 so its doing as little as possible which is why bin/ci and config/ci are just being removed - we might want to actually be using them as a way of running everything locally, but first someone needs to explore having them updated to reflect our actual CI stuff.

config/bundler-audit.yml however is just something we'll never need as use osv-detector & osv-scanner for security auditing