Fine-tune dependabot config

[teutoburg]

I put our own dependencies in a separate group, since we regularly forget to upgrade those.

I'd like to try to be reminded on a monthly basis (not too much spam) about new major and minor versions of our (very few) main dependencies. If that gets too annoying still, we can always switch it off again.

I'm not 100 % certain if we will now get any other PRs as well, perhaps patch level version updates need to be excluded explicitly, we'll see. In any case, the security updates will go through.

I'd like to keep this (perhaps again fine tuned) config for ScopeSim now, and reevaluate in a few months time if and how we'd like to roll that out to our other repos.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.85%. Comparing base (b5257b7) to head (69245c4).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #591   +/-   ##
=======================================
  Coverage   76.85%   76.85%           
=======================================
  Files          69       69           
  Lines        8571     8571           
=======================================
  Hits         6587     6587           
  Misses       1984     1984           

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[teutoburg]

I'll merge this to avoid two more PRs tomorrow. It can't break anything functional anyway, worst thing that can happen is more PR spam from the bot 🙃