Add dependabot.yml

[jsangmeister]

E.g. the autoupdate service has the following file: https://github.com/OpenSlides/openslides-autoupdate-service/blob/main/.github/dependabot.yml. I think this would be a good addition to the backend and would take over work which we currently do manually. Also, it eases the update workflow because each updated is applied for itself and not in a batch with all other updates.