test: Remove support for Node 18

[BilalQamar95]

Description

Completed upgrade to Node 20 by removing the Node 18 CI check and using .nvmrc for version to use.

See the tracking issue for further information.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.60%. Comparing base (a703aba) to head (784598a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #536   +/-   ##
=======================================
  Coverage   70.60%   70.60%           
=======================================
  Files          24       24           
  Lines         364      364           
  Branches       96       96           
=======================================
  Hits          257      257           
  Misses        105      105           
  Partials        2        2           

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[brian-smith-tcril]

These changes look great! I'm going to hold off on merging this along with all other PRs dropping Node 18 until Sumac is cut as per openedx/public-engineering#280