The majority of users should have already switched or upgraded to Ubuntu 20.04 at this point, so there is no point in building further Ubuntu 18.04 versions. This saves us some CI slots, and also reduces the number of target systems to actually support.
Implements a manual and automatic update checker with support for both release and testing update channels, allowing users to stay as up to date as possible. It is fully compliant with privacy regulations around the world, as it stays completely silent and inactive until the user gives the Ok to connect to GitHub for the latest releases.
Ubuntu 20.04 has officially replaced 19.10 as the latest version, and is stable enough that people can move to it. The script doesn't change much for that. However we do remove the needless extra builds here and revert to the system installed clang version, which thanks to other pull requests now works fine. For Ubuntu 18.04 that is clang-8, while for Ubuntu 20.04 it is clang-9.
Crowding commits each language in its own commit, instead of merging multiple into one commit. This results in very spammy builds, to the point of several hundred being spawned in the same second.
Integrates Github Actions which is much much faster than AppVeyor in all areas, and even supports multiple workflows instead of forcing everything into just one workflow like AppVeyor does. Plus we get 20 parallel builds that nearly instantly finish, which results in much faster feedback without having to run our own Jenkins CI.
The builder and packager scripts have been adjusted to add support for both Windows and Linux, and both AppVeyor and Github Actions. Additionally to that, the builder script now correctly executes x32 and x64 steps in a chain, instead of waiting for the other architecture to finish first. This further reduces build times.
