Rename default "master" branch to "main"

[fnogatz]

I would like to suggest the renaming of the project's default branch master to main, following Git's defaults. A brief summary of the recent developments regarding the shift towards main instead of master is given in these articles of GitLab and GitHub. I think only @henrikingo currently has the permissions to do so. Any chances we can change this?

[henrikingo]

Yes, I agree now is a good time. I would propose if you finish some of your other work first, I will add you to the admin group, so that you can do this task yourself. I think mainly you should then double check that the CI still works correctly.