reducing the entropy of the commands to play with apptainer

I have removed recently introduced commands such as scram and cmsenv, as their multiplicty started to be counterproductive (I expect an interactive rebase in the migration branch before merging to master). More and more commands would become necessary, with a lot of redundancy. Instead, a prefix command, das-cmssw, has been introduced, and can be run as follows from EL9:

das-cmssw el8 scram b -j8
das-cmssw el8 dasgoclient -query "/QCD*/*20UL18*/MINIAODSIM"
das-cmssw el8 cmsRun Ntupliser_cfg.py
das-cmssw el8 crab -h

In particular, dasgoclient is used in mkNtuples (see Core!170 (merged)).

Also, the cc7 and el8 command are not executing their arguments as a few weeks ago. I haven't identified the issue, but decided to simply remove this possibility.

I have also added instructions to use of the three remaining commands.

One slight caveat is that there is still some redundancy in the scripts of the respective commands. Furthermore, some further specialisation to CERN or to DESY is expected. We should consider in an upcoming MR to improve these two aspects, perhaps with the help of CMake.