Change default branch to main

[timsueberkrueb]

Checklist:

• Temporarily create main branch and check that CI runs through (ensure no release-related workflows run before doing so)
• Use the GitHub rename branch feature to rename master to main
  This should make the following steps redundant (do verify before proceeding):
  • Change default branch in GitHub settings to main
  • Change target branch of all open PRs from master to main
• Merge this PR into main
• Execute migration in the other repositories: effekt, effekt-website

Note

I temporarily created a main branch to verify the CI runs through. I removed the branch again to prevent the release workflow from potentially running twice.