Upgrade server to Ubuntu 24.04

[willgearty]

The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-01 actions/runner-images#11101. We'll need to do one of the following soon:

1. Upgrade our dependencies to work with Ubuntu ≥ 22.04
2. Use a self-hosted runner

I'm not sure how much work #1 would take (the last time we upgraded Ubuntu was in #3195) and could also require that we update the AWS server and our dev VMs.
#2 could work well if we can just use Hipparchia, but it's also not clear to me how much work that would require (and how much longer should we be using Ubuntu 20.04 anyways, since April 1 represents the EOL).

[willgearty]

Oh, actually, we upgraded the dev VMs to 24.04 in #3616. So upgrading the tests should be straightforward. But we should still upgrade Hipparchia to 24.04 in the near future.

[willgearty]

The tests are now upgraded to 24.04!