Change default branch to main

As part of Rename default branch of existing project templ... (gitlab-org/gitlab#428020 - closed), let's change the default branch to main.

1. Review whether the current default branch is additionally used (for example, by .gitlab-ci.yml or by Container Registry)
2. Create main branch from the existing default branch
3. Set main branch as the default one
4. The previous default branch can be left in the repo. We can clean them up during a major release (even though it's unlikely necessary).