Open pull requests in all related repositories after an update

[jsangmeister]

After this repo has been updated, pull requests should be opened automatically in all relevant repos (currently backend, client, autoupdate and search service) which update the respective submodule. This way, it is ensured that

1. the maintainers of the repo are informed about the changes
2. all repos always share the same version of the meta repo (assuming that most of the time the PR can simply be merged directly), which can be checked in the main repo
3. the overhead of updating the modules and creation of PRs is kept as low as possible.

Sometimes, manual intervention will of course be necessary, but this should be the regular case.

[jsangmeister]

Open question: Should a PR be created for every new commit in the repo or only the ones changing {models|permission}.yml? If we do the latter, it will happen that e.g a dependency in the meta repo is updated and the backend updates the same dependency together with the meta repo. Then, the backend is ahead of the other service regarding the meta repo commit and all other services must be updated manually if an update should be built (which means creating PRs in every service, waiting for the CI and maybe approvals, merging it and then being abel to update the main repo). However, if we do the former, it might lead to a lot of unnecessary PRs, e.g., when dependencies get updated. For example, the autoupdate service doesn't care about an updated formatter in the meta repo.