Auto-approve and merge most dependabot PRs

[dontrolle]

Is your feature request related to a problem? Please describe.
Currently, we need to approve and merge dependabot PRs ourselves. In the case of patch or minor version updates, this is mainly a case of just waiting to see whether CI has run through w.o. problems.

Describe the solution you'd like
Let's just automate approval and merge of (at least) dependencies, where it is just a bump of a patch or minor version. May consider automerging also major version updates.

There are user-created Actions, which help with some of these, or we could adjust some of the solutions in the GH Actions docs: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions