Use the lhcb/Gaudi fork for cmake-modernisation
Currently, we use the cmake-modernisation branch from https://gitlab.cern.ch/clemenci/Gaudi. Here I propose to use the lhcb/Gaudi fork instead, so that we can make progress on lhcb-dd4hep, lhcb-new-cmake and lhcb-gaussino while @clemenci is away. The first step would be to rebase it on (or merge it with) Gaudi/master.
Anyone on LHCb will be able to push to cmake-modernisation in lhcb/Gaudi as the branch is not protected. If we don't like this we can protect it and change it with merge requests, but we may need to extend the list of maintainers in that fork.
I also removed the temp slot for testing the new job options service (not needed any more).