Move `master` branch to `main`

Cc @schloerke

The master branch of this repository will soon be renamed to main, as part of a coordinated change across several GitHub organizations (including, but not limited to: tidyverse, r-lib, tidymodels, and sol-eng). We anticipate this will happen by the end of September 2021.

That will be preceded by a release of the usethis package, which will gain some functionality around detecting and adapting to a renamed default branch. There will also be a blog post at the time of this master --> main change.

The purpose of this issue is to:

• Help us firm up the list of targetted repositories
• Make sure all maintainers are aware of what's coming
• Give us an issue to close when the job is done
• Give us a place to put advice for collaborators re: how to adapt

message id: entire_lizard

Es werden keine Dateien oder Tests genannt. Beginne damit, den aktuellen Standard-Branch des Repositorys und die referenzierte usethis-Anleitung zu prüfen, und bestätige anschließend das Ziel-Repository sowie die im Issue beschriebene Abstimmung mit den Maintainer:innen. Erledigt bedeutet, dass die Migration des Branches abgestimmt ist und die Hinweise der Collaborator:innen festgehalten sind.