GitHub Ubuntu-20.04 runner no longer supported #317

From what I understand, the CI workflows referencing ubuntu-20.04 will either fail or never run (I think that's why I'm seeing The job has exceeded the maximum execution time while awaiting a runner for 24h0m0s) because GitHub Actions dropped support for Ubuntu 20 as of April 2025: https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

I could bump the image as part of #316 but (per hdgarood's comment) it will need to match the version of Ubuntu that's used wherever this is deployed, so would need input from someone that has access to the deployment.