gitlab-ci: shorten job prefixes in 3.22 release branch

This helps to maximize the amount of information visible in the GitLab web interface.

Also document their meaning in the developer documentation and in the CI configuration file directly.

This backports !6744 (merged) to the release branch, and adds a maintainer guide documentation update missed in that MR.

See: https://gitlab.com/gitlab-org/gitlab/-/issues/8496
Backport: release:HEAD^2