Switch default branch from "master" to "main"

[hjacobs]

As mentioned in #1365: we want to switch from master to main.

We already had new PRs to main: #1367 #1366

@RobbeSneyders I think I can just change the default branch in GitHub to "main", so the repo view looks "correct" (e.g. updated CI/CD badge). Are you OK with switching the default branch?

[RobbeSneyders]

Sounds good to me.

As far as I understand, Zappr is automatically applied to the default branch though, so we might want to update the Zappr config first.

[hjacobs]

Switched to main.