move script to main branch

We started with a branch called linux-debian11, but decided in discussions that we shouldn't do that, and instead put anything that is operating system dependent into the script.

The task is thus to

• rename the linux-debian11 branch into main,
• make main the default branch in the gitlab project, and
• update any merge requests to want to merge to main instead of linux-debian11. The third item may happen automatically.