Rename default branch to "main"

[sbueringer]

Hey folks,
we're going to rename the default branch from master to main (for consistency with other project like e.g. controller-runtime)

PRs:

• CT: 🏃 Rename default branch to main #1038
• test-infra: controller-tools: change branch to main kubernetes/test-infra#33323

[sbueringer]

Branch has been renamed. You can run these commands to adapt locally

git branch -m master main 
git fetch origin 
git branch -u origin/main main 
git remote set-head origin -a