We drop debian-jessie (see #25009) and add the latest versions of debian, ubuntu, linuxmint, fedora.

We also remove the separate workflow for testing platform variants using the gcc_spkg configuration factor. (tox-optional.yml and tox-experimental.yml continue to test the platforms on which gcc_spkg is helpful in order to build optional packages: centos-7-gcc_spkg and centos-7-i386-gcc_spkg.)

CC: @dimpase @kliem

Component: porting

Author: Matthias Koeppe

Branch/Commit: ac47e7d

Reviewer: Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/32947