feat(gitlab-ci): add `ubuntu-22.04` [skip ci]

Update recently merged into the salt-bootstrap repo:

• https://github.com/saltstack/salt-bootstrap/pull/1820

Ubuntu 22.04 was released on 2022-04-21:

• https://en.wikipedia.org/wiki/Ubuntu_version_history#Ubuntu_22.04_LTS_(Jammy_Jellyfish)

Also includes:

• Add g++ package for git build(s), as initially done with Fedora 35 in e2e19dbd
• Install contextvars until package fixed, as done with bookworm recently in d80b2bf3