Add dependabot auto-merge action

[slifty]

This PR adds an action to set Dependabot PRs to auto-merge.

Since we have strong branch protections this is OK: dependabot PRs will still require human approval and CI must still pass, but now we don't have to manually click the "merge" button after approval.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.93%. Comparing base (7beb31a) to head (348a823).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #804      +/-   ##
==========================================
- Coverage   46.95%   46.93%   -0.02%     
==========================================
  Files         372      372              
  Lines       11444    11444              
  Branches     1896     1896              
==========================================
- Hits         5373     5371       -2     
- Misses       5882     5885       +3     
+ Partials      189      188       -1     

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