Ubuntu Focal 20.04 has reached its EOL and the runner image in the CI already became unavailable (see b9e67335ce73025df49bb84bc84796b2cc9b32b8). - [x] Remove Ubuntu 20.04 setup and configuration instructions from the [Developer Guide](https://github.com/Ericsson/CodeCompass/blob/master/doc/deps.md). - [x] Remove Ubuntu 20.04 jobs from the CI.
Ubuntu Focal 20.04 has reached its EOL and the runner image in the CI already became unavailable (see b9e6733).