Rename branch master to main

[JanWittler]

As a step towards more inclusive naming, it is recommended to avoid the master / slave terminology and rather name the default branch from now on main. Most major GIT repository providers already changed their default branch name to main for new repositories.
I would suggest to follow this naming improvement. As this change will require local copies to be updated, it might make sense to combine it with #496, #497.

• Rename master to main branch on all Vitruv repositories
• Change repository default branch to main
• Enforce branch protection rules for main
• Update CI workflows to use main branch
• Update links in Readme to reference branch main

GitHub already provides support for the first three bullet points (https://github.com/github/renaming).