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 58.22%. Comparing base (db01cde) to head (17b8204).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #206   +/-   ##
=======================================
  Coverage   58.22%   58.22%           
=======================================
  Files          25       25           
  Lines         395      395           
  Branches       66       66           
=======================================
  Hits          230      230           
  Misses        149      149           
  Partials       16       16           

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[coveralls]

Pull Request Test Coverage Report for Build 10772705447

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 55.473%

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Totals
Change from base Build 10741783659:	0.0%
Covered Lines:	168
Relevant Lines:	305

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💛 - Coveralls

[kdmccormick]

We are closing this pull request because Program Console is deprecated. If you work with 2U and you still want to make this change, you could propose it on 2U's fork of Program Console, but the contribution will not be part of the Open edX project.