Move from Debian Wheezy to Debian Buster for our Linux builds

Debian Wheezy is about to get unsupported and we should move to Debian Jessie for our Linux builds. This has the additional advantage that we don't have different Debian versions anymore to build bundles for all of our supported platforms: We are then using Debian Jessie everywhere.

The only worrying situation is the CentOS one. We should think about whether we still can and want support CentOS 6 (which we need to do anyway while switching to Firefox ESR 60 which requires GTK3) and what the CentOS 7 situation is if we start building using Jessie.