CI: disallow "squash commits" checkbox, because it messes up the commit message

As seen in !373 (merged), having commits squashed changes the commit message. This is bad, because we go through the trouble and add the MR-ID (e.g. (!373)) to each commit (see mrhlpr). So we don't want gitlab to mess up the message.

Most of the time it is obvious, when commits should be squashed anyway, so the person who does the merge (currently @MartijnBraam and me) could squash them for themselves, with git rebase -i and changing pick to squash.

Adding this to CI is pretty easy actually, just modify check_mr_settings.py accordingly, which is already testing another gitlab MR option: https://gitlab.postmarketos.org/postmarketos/pmaports/blob/master/.gitlab-ci/check_mr_settings.py

TODO:

• adjust pmaports
• adjust pmbootstrap

Any takers (maybe from @postmarketos)?