Stick to `ubuntu:20.04` runners for now, as porting to newer ones implies
fixing a number of compilation warnings which may not be worth the trouble
for an EOL branch.
It's too hard to get compatibility between GNU and BSD sed,
so let's just use perl oneliners.
And improve it to also remove trailing tabs, not just spaces.
(cherry picked from commit c71e189efd)
Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).
Backports new style scripts from `master` branch too to do the same checks.
