ci: ensure test jobs also run on the default branch

I hesitated between adding extra syntax to ensure they always run for the default branch. Or completely removing the changes syntax.

I opted for the latter, since:

1. We introduced the changes syntax to reduce the runner minutes of shared GitLab runners. But we don't use these runners anymore.
2. The syntax of the CI is simpler and more straightforward now.

Issue: #39 (closed)