Reduce number of dependabot PRs

[glatterf42]

This PR adapts the dependabot config file to reduce the number of PRs created by bumping versions. It does so in two ways:

1. Group all dependencies together so that version updates will only be one PR
2. Decrease the schedule.interval to monthly

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• ~[ ] Add or expand tests; coverage checks both ✅~ No functionality added.
• ~[ ] Add, expand, or update documentation.~ Nothing user-facing added.
• ~[ ] Update release notes.~ Nothing user-facing added.

[codecov[bot]]

Codecov Report

Merging #756 (851440c) into main (e40c347) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #756   +/-   ##
=====================================
  Coverage   94.3%   94.3%           
=====================================
  Files         43      43           
  Lines       3474    3474           
=====================================
  Hits        3279    3279           
  Misses       195     195           

[glatterf42]

@khaeru, If you don't find time until Friday to review this, I will just merge this without approval since next dependabot PR would be due next Monday with our current configuration.