CI: Upgrade to codecov 4 & use token.

[JacobOaks]

This upgrades codecov CI action to version 4,
and uses the codecov token when uploading,
since tokenless uploading is no longer supported for some cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.14%. Comparing base (31095c6) to head (039f6e4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   98.45%   99.14%   +0.69%     
==========================================
  Files           6        6              
  Lines         323      234      -89     
==========================================
- Hits          318      232      -86     
+ Misses          4        1       -3     
  Partials        1        1              

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