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 94.98%. Comparing base (cf5e1a6) to head (5aba677).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #408   +/-   ##
=======================================
  Coverage   94.98%   94.98%           
=======================================
  Files         139      139           
  Lines        1356     1356           
  Branches      265      265           
=======================================
  Hits         1288     1288           
  Misses         60       60           
  Partials        8        8           

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[farhaanbukhsh]

@BilalQamar95 Sumac is done :) maybe if you can rebase this PR we can merge this in.