Add horizon.ci.harmonia option and bump gitlab-ci to 0.21.0
Summary
- Add
horizon.ci.harmoniaoption withenableandstore(defaults tossh://harmonia@cache.horizon-haskell.net) - Add local
ci/flake.nixpinninggitlab-cito 0.21.0 (includes harmonia module) - Changelog entry for 0.24.0