Update Deprecated GitHub Action Runner

Update deprecated GitHub Action Runner [Detectree docker CI test](https://github.com/PatBall1/detectree2/actions/workflows/docker-test.yml).

Currently, it cancels the run with the following message:

> This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner will be removed on 2025-04-15. For more details, see https://github.com/actions/runner-images/issues/11101