Drop unmaintained codecov.io dependency

[nsoranzo]

Also:

• Update package-lock.json with npm install --package-lock-only
• Test on all branches and pull requests.
• Test on oldest supported node version 20.
• Cache npm dependencies.
• Update packages with vulnerabilities with npm audit fix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.03%. Comparing base (3225d22) to head (f8aa560).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #125   +/-   ##
=======================================
  Coverage   41.03%   41.03%           
=======================================
  Files          51       51           
  Lines        1974     1974           
  Branches      441      441           
=======================================
  Hits          810      810           
  Misses       1047     1047           
  Partials      117      117           

Flag	Coverage Δ
unittests	41.03% <ø> (ø)

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[nsoranzo]

After this, all security advisories and dependabot alerts closed!