Update the default operating system images

[snay2]

Describe the feature
simple-ec2 suggests AMIs for several different operating system types (Amazon Linux, Windows, Ubuntu, etc.). Some of these may be outdated now. One particular example is Ubuntu: we are using 18.04 LTS, but there is a more recent long-term support version: 20.04. We should use that instead, and see if any of the other operating system versions ought to be bumped up as well.

(Or if wait for another month or two, we can upgrade directly to Ubuntu 22.04 LTS, which should be released in April 2022.)

Is the feature request related to a problem?
Using more recent OS versions will give users the latest features and security fixes. The biggest advantage is that the software repos (available via apt, yum, etc.) will have more recent versions of software users are likely to want to install once they get their instance.

Describe alternatives you've considered
Users can manually supply an AMI they prefer.