ci: downgrade codecov action to v3

[G-Rath]

It seems that v4 is having some teething problems - coverage is no longer being uploaded due to the lack of a token, and it seems like at least some people are getting 500s even when they do provide a token (codecov/codecov-action#1248).

Let's start by downgrading to a known working version, and go from there.

Relates to #784

Before:

After:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.18%. Comparing base (b28c1c8) to head (01cb733).
Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #871      +/-   ##
==========================================
+ Coverage   59.78%   61.18%   +1.39%     
==========================================
  Files         136      144       +8     
  Lines       11268    11808     +540     
==========================================
+ Hits         6737     7225     +488     
- Misses       4102     4110       +8     
- Partials      429      473      +44     

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