Switch default branch to main

We want to switch the default branch name from master to main for all gridscale Open Source projects including this repository.

This involves:
- Switching the default branch name here on GitHub
- Updating all references of the master branch name in GH Actions and other CI/CD pipelines and scripts
- Giving contributors a bit of info on how to update their remotes in their local copies of the repo

Would be nice to do this coordinated and in "one go" so that contributors are not surprised by inconsistent naming or repositories not having switched yet. If you want to help out here contact @bkircher