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 84.14%. Comparing base (59dbee3) to head (21f5bbc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #257   +/-   ##
=======================================
  Coverage   84.14%   84.14%           
=======================================
  Files          47       47           
  Lines         700      700           
  Branches      133      136    +3     
=======================================
  Hits          589      589           
  Misses        111      111           

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