appimagetool: Prevent AppImages from being created if AppDir ingredients are too new

Many AppImages get made using too new ingredients (and not bundling _everything_), resulting in those AppImages not being able to run on all still-supported LTS releases of major Linux distributions.

Personally, I am sick and tired of having to explain over and over again that you should not generate AppImages that cannot run on at least all still-supported LTS releases of major Linux distributions. It is eating way too much of my time, and it is giving the AppImage format a bad reputation ("doesn't work as advertised").

We need to somehow prevent such AppImages from being created in the first place. The low-level AppImage creation tools should simply print an error message and refuse to continue. Similar to how linuxdeployqt refuses to produce such AppImages.

Reference:
* https://github.com/AppImage/appimage.github.io/search?q=glibc&type=issues