Group dependabot PRs; schedule monthly

[rkingsbury]

Group dependabot PRs and set them on a predictable monthly cadence.

Previously, dependabot PRs would arrive separately (and generate separate PRS) for each individual dependency.

See documentation at https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/optimizing-pr-creation-version-updates

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.02%. Comparing base (bad292b) to head (f1ee646).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1088   +/-   ##
=======================================
  Coverage   70.02%   70.02%           
=======================================
  Files          47       47           
  Lines        4064     4064           
=======================================
  Hits         2846     2846           
  Misses       1218     1218           

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