Rename the default branch from "master" to "main"

[mmerklinger]

As discussed a while ago, we want to unify the default branch over all repositories to main.

This are the tasks to be done before.

• Review old PRs and branches targeting master.
• Change mentions of master in GitHub Action workflows.
• Documentation, hyperlinks, or other mentions of the master branch.
• Rename master to main.
• Update default branch and branch protection rules in GitHub settings.

Please edit if you have more points!