Disallow Squash Merge

In the PL circle it was decided to technically enforce that only Rebase & Merge is allowed in our repositories to avoid that e.g. commits from different authors are squashed by accidident (https://github.com/orgs/eclipse-score/discussions/1685). A squash could still be done locally, if the requirements are fulfilled to do so. Therefore, the otterdog config has to be adjusted. @qor-lb and me looked into it, but we were not completly sure because of the different functions `newScoreRepo` and `newRepo`. @AlexanderLanin, could you please have a look?