CI: `ubuntu-18.04` runner image is deprecated

Theis part of CI appears to rely on the ubuntu-18.04 image label:

nwipe/.github/workflows/ci_ubuntu_18.04.yml

runs-on: ubuntu-18.04

However, this runner image is deprecated:
The Ubuntu 18.04 Actions runner image will begin deprecation on 8/8/22 and will be fully unsupported by 4/1/2023

Perhaps it's time to get rid of ci_ubuntu_18.04.yml.

Agreed. Are you offering to update the CI 18.04 file to 20.04? If so please go ahead, that would be appreciated. 👍

Note that:

file ci_ubuntu_latest.yml already exists,
ubuntu-latest is currently an alias for ubuntu-20.04.

I could rename ci_ubuntu_latest.yml to ci_ubuntu_20.04.yml and create a ci_ubuntu_22.04.yml file too. But then we lose the benefit of automatically tracking the latest Ubuntu "latest" version (as defined by GitHub).

I would recommend just getting rid of .github/workflows/ci_ubuntu_18.04.yml as a first step.

Sounds good, go for it 👍