Use a newer version of Ubuntu for the Terraform in this repository

[aiyengar2]

Currently, we use Ubuntu 18.04. However, we should update our tests to use a newer version of Ubuntu (or SLES) by default since Ubuntu 18.04 is no longer under standard support as of June 2023: https://wiki.ubuntu.com/Releases

We should switch to 20.04 for now.

[aiyengar2]

We should also bump the Docker version in this repository accordingly, see #226.