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.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.27%. Comparing base (e0a2a9c) to head (23ab805).
Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3237   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         249      249           
  Lines        4401     4401           
  Branches     1037     1037           
=======================================
  Hits         4105     4105           
  Misses        293      293           
  Partials        3        3           

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