ちょっと調べてみたらRuby 3で使えるRailsは6.1からだったね（白目）
Ruby & Rails Compatibility Table - FastRuby.io | Rails Upgrade Service https://fastruby.io/blog/ruby/rails/versions/compatibility-table.html
fastruby.ioRuby & Rails Compatibility Table - FastRuby.io | Rails Upgrade ServiceThis is a short post to show the compatibility between Ruby on Railsand Ruby across different versions. In theprocess of upgrading really old applications to more modern versions of Ruby andRails we have ran into a lot of these combinations.