test: Remove support for Node 20

[bydawen]

Description

Complete the upgrade to Node 24 by removing the Node 20 CI check and going back to using .nvmrc as the source of truth for which version to use.

See the tracking issue for further information.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.48%. Comparing base (899d1fa) to head (05708fc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #811   +/-   ##
=======================================
  Coverage   94.48%   94.48%           
=======================================
  Files         167      167           
  Lines        3699     3699           
  Branches      999     1003    +4     
=======================================
  Hits         3495     3495           
  Misses        190      190           
  Partials       14       14           

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