Update all GNU docker images to latest LTS version on Docker.#591

Merged

bors[bot] merged 2 commits intocross-rs:mainfrom Alexhuszagh:increment_versions

Jul 16, 2022

+271-158

Commits on Jul 15, 2022

Increment Ubuntu base image versions to 20.04.
Alexhuszagh
committed

Commits on Jul 16, 2022

Remove Linux image from mips-unknown-linux-gnu.
Alexhuszagh
committed