Group dependabot PRs; monthly schedule

[rkingsbury]

Summary

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 86.10%. Comparing base (12b4e8b) to head (18596a4).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #379   +/-   ##
=======================================
  Coverage   86.10%   86.10%           
=======================================
  Files          14       14           
  Lines        1821     1821           
  Branches      313      313           
=======================================
  Hits         1568     1568           
  Misses        209      209           
  Partials       44       44           

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