Merge tag 'comoproj-v2.6.2' into pre-release/comoproj/coqc-8.16.1-ocaml-4.13.1

Added explicit licensing to every file

The Makefile is now explicit licensed under GNU GPLv3 with copyrights to
Max Ole Elliger and Philip Kaludercic.

The .gitlab-ci.yml is now explicit licensed under GNU GPLv3 with
copyrights to Max Ole Elliger.