docs: update badge URL

[iuioiua]

This change deletes the local badge file and instead calls directly to shields.io. It'll break only a few spots that use the current badge, but the note should help them figure out how to update.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.24%. Comparing base (a81742a) to head (186ac2b). Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5787      +/-   ##
==========================================
- Coverage   96.26%   96.24%   -0.02%     
==========================================
  Files         481      479       -2     
  Lines       38738    38709      -29     
  Branches     5615     5615              
==========================================
- Hits        37291    37256      -35     
- Misses       1406     1411       +5     
- Partials       41       42       +1     

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