Use DEB822 for Ubuntu Install Guide

[tcyrus]

The DEB822 format (.sources files) has been supported since apt 1.1 and has been the default format since Ubuntu 24.04 LTS.

Ubuntu 20.04.6 LTS ships with apt 2.0.9 according to the manifest file, so I think it makes sense to update the "Ubuntu 20.04 and above" section of the install guide to use the "new" repo file format.

# Add the repository.
export CODENAME="$(lsb_release -cs)"
sudo tee /etc/apt/sources.list.d/rethinkdb.sources <<-EOF
Types: deb
URIs: https://download.rethinkdb.com/repository/ubuntu-$CODENAME
Suites: $CODENAME
Components: main
Signed-By: /usr/share/keyrings/rethinkdb-archive-keyrings.gpg
EOF