v0.0.21 Release

This Releases most notably adds support for two new Ubuntu releases (Focal, Eoan), Indexer datamodel refactor, UpdaterSets to hold groups of Updaters, and various logging/bug fixes.

Documentation for this release can be found here: https://quay.github.io/claircore/v0.0.21/

#169 Louis/layer scanned fix louis/layer-scanned-fix
#168 Add default update sets louis/add-default-update-sets
#167 Louis/bump ubuntu releases louis/bump-ubuntu-releases
#166 add consistent linux bridge name louis/firewalld-docker-compose
#165 postgres: defer after checking error hank/segfault
#164 implement updater sets louis/updater-set
#162 Indexer datamodel refactor louis/indexer-dm-refactor
#160 Ditch Ubuntu Updaters louis/ditch-ubuntu-updaters