Switch from master to main

[christian-bromann]

Let's abandone non-inclusive terms such as master, slave, blacklist, and whitelist and therefor switch the branch name to main. This should be a fairly straight forward process, just rename the branch in the settings. All PRs will be automatically updated to target the new default branch.

[christian-bromann]

This doesn't seem to be straightforward but there is a workaround used in here: Homebrew/brew#7746 (comment)