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.
Ensures that we have a configuration that is currently usable.
Some changes for the `3.2` branch as it's unmaintained:
- No cache
- No artifacts upload
- Uses clang-format 10
- No JavaScript linting
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.
