Change CI/CD to impish
Update the metadata to the new development release "Impish Indri"
Reduce the number of builds to the really neccessary builds:
- ci/unstable builds make sense only for the development release
- ci/stable builds make sense only for the development branch and the LTS versions
When more releases are needed, they can be added easily.
Test Plan: Check that packages for impish are built
Reviewers: tsimonq2, kc2bez, teward
Reviewed By: kc2bez, teward
Differential Revision: https://phab.lubuntu.me/D110