Drop about-to-be-removed Ubuntu 20.04 from CI (fixes #282)#283

vitalybuka merged 2 commits intogoogle:mastergoogle/libprotobuf-mutator:masterfrom

hartwork:drop-end-of-life-ubuntu-20-04-from-cihartwork/libprotobuf-mutator-fork:drop-end-of-life-ubuntu-20-04-from-ciCopy head branch name to clipboard