Autoproj v2.13.0 and Autobuild 1.21.0 have been released

A rather small update:

The main piece of work is fixed compatibility and most warnings for Ruby 2.7 (main Ruby version on Ubuntu 20.04)

One new functionality: created an interactive update, autoproj update --ask, which asks for each modified packages whether the package should be updated, after showing the status, that is the new commits locally and remote (Note: it does not yet cover the package sets, only the packages themselves).

This allows to update knowing what happened: