src/source/software.rst: suggest using "apt full-upgrade" instead of "apt upgrade"

The difference between "apt upgrade" and "apt full-upgrade" is, that the latter is allowed to remove packages and old kernel images. The former is recommended for Debian stable systems where software updates are usually only security fixes which do not require any package removal when applying them. Thus, the "apt upgrade" command is meant as a less disruptive method (because it cannot remove packages) to keep a Debian stable system up-to-date. Users would usually run "apt full-upgrade" when they want to upgrade from one Debian stable release to the next which does involve plenty package removals.

When using Debian unstable, changes in package dependencies require package removal regularly and an upgrade of a Debian stable system is thus similar to doing an upgrade from one release to the next. Without allowing removal of packages, the upgrade will often not be able to completely upgrade the whole system and leave some packages in their old version. Furthermore, the mechanism to remove old kernel package versions will not be executed for an "apt upgrade" as that is not allowed to remove packages.

Since the MNT Debian flavour is based on Debian unstable, the handbook should recommend the use of the more dangerous but more appropriate "apt full-upgrade" instead of "apt upgrade".