CI: Drop CircleCI, using GHA only

[effigies]

[codecov[bot]]

Codecov Report

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

Project coverage is 83.66%. Comparing base (82f419b) to head (2b7e23e). Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
+ Coverage   76.34%   83.66%   +7.31%     
==========================================
  Files          32       32              
  Lines        2816     2816              
  Branches      377      377              
==========================================
+ Hits         2150     2356     +206     
+ Misses        598      390     -208     
- Partials       68       70       +2     

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[effigies]

I'm going to merge to fix the builds. It might be worth re-adding CircleCI config just for the artifacts, but triggered on a [circleci] tag in the commit message or something.