Enable grouped dependabot updates

[timobrembeck]

Motivation

GitHub released grouped version updates for Dependabot in public beta now. I think this it's worth checking out the new functionality.

Also see config docs: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups

Proposed Solution

Test wether grouped dependabot updates work well for our repository. I think once a month or bi-weekly would be a sensible interval.

Alternatives

Just do manual updates