Matrix was removed from Ruby’s standard library in version 3.1. More info: https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/
With Ruby 3.1, matrix needs to be explicitly added to the Gemfile. You can add it manually or run something like:
$ bundle add matrix
After it’s added to the Gemfile, bundle your application:
$ bundle install
Then your application should continue to behave like it did in previous Ruby versions.