Change CI/CD to impish

Summary:
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