So I have a past project from 2014 that still running until now (5 independent services: 3 written in Golang - 2 read-only service, 2 write-only service written in Ruby, 1 PostgreSQL, 1 Redis, ~45GB database / 2.7GB daily backup, 1.1GB cache, 8GB logs, <800MB compressed), and the server RAID already near death (since the server was alive without issue for 12 years), and on new server there's freshly installed 22.04, and old service when trying to be run with ruby 3.x will always failed because of library compilation issues (eg. still depend on outdated postgres12-dev). Here's how you can install ruby 2.8 without rvm:
# add old brightbox PPA (must be focal, not jammy)
echo 'deb https://ppa.launchpadcontent.net/brightbox/ruby-ng/ubuntu focal main
deb-src https://ppa.launchpadcontent.net/brightbox/ruby-ng/ubuntu focal main ' > /etc/apt/sources.list.d/brightbox-ng-ruby-2.7.list
# add the signing key
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys F5DA5F09C3173AA6
# add the focal security for libssl1.1
echo "deb http://security.ubuntu.com/ubuntu focal-security main" | sudo tee /etc/apt/sources.list.d/focal-security.list
# update and install
sudo apt update
sudo apt install -y ruby2.7 ruby2.7-dev
that's it, now you can run gem install (with sudo for global) without any compilation issue as long as you have correct C library dependency (eg. postgresql-server-dev-12).
