ci: refer to default branch instead of hard-coded branch name

[olavis]

Describe Problem

The default branch is hard-coded in some of the workflows in the template - branches: - main. Some users of the template might wan't to use master (or other) instead of main, and it would be nice if github actions just worked regardless.

Suggest Changes

In github actions, use [ $default-branch ] to refer to the default branch in the repo.