Make it easier to specify the "main" branch for tagging script

[tmadlener]

The tagging script is geared towards master being the main branch of a repository. github has changed its default to main quite some time ago, and the tagging script can be used to target that, but it is a bit cumbersome, e.g.

ilcsofttagger.py --packages key4hep/k4SimDelphes/v00-01-08/main 

Ideally this would work the same as for repositories that have master as their main branch, where it is enough to do

ilcsofttagger.py --packages key4hep/k4SimDelphes

It is not a huge problem, but maybe we have some spare cycles to do this at some point.

[andresailer]

Can get the default branch via the API: https://stackoverflow.com/a/16501903
So should be fairly easy to do.