test: Remove support for Node 18

[Anas12091101]

Complete the upgrade to Node 20 by removing the Node 18 CI check and going back to using .nvmrc as the source of truth for which version to use.

See the tracking issue for further information.

[openedx-webhooks]

Thanks for the pull request, @Anas12091101!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (567a020) to head (ca6fcbc).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #352   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          111       111           
  Lines         1082      1082           
  Branches       159       159           
=========================================
  Hits          1082      1082           

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

[mphilbrick211]

@brian-smith-tcril - is this something you can help review?

[Anas12091101]

@brian-smith-tcril, could you please review this PR when you have a moment?

[brian-smith-tcril]

The plan is to not drop Node 18 support until after Sumac is cut. The changes in this PR look perfect but I don't want to approve the PR and have someone accidentally merge it early.

See the DEPR for more details on this plan: openedx/public-engineering#280

[Anas12091101]

Thank you for the clarification👍