test: Remove support for Node 18

[BilalQamar95]

Note: As per openedx/public-engineering#280 this can merge after Sumac cut.

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 58.61%. Comparing base (4136a63) to head (8576f7a).
Report is 33 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1112   +/-   ##
=======================================
  Coverage   58.61%   58.61%           
=======================================
  Files         117      117           
  Lines        2317     2317           
  Branches      639      641    +2     
=======================================
  Hits         1358     1358           
  Misses        898      898           
  Partials       61       61           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.