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 83.26%. Comparing base (1fd3343) to head (07bb5df).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #213   +/-   ##
=======================================
  Coverage   83.26%   83.26%           
=======================================
  Files          47       47           
  Lines         693      693           
  Branches      135      133    -2     
=======================================
  Hits          577      577           
  Misses        116      116           

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

[Anas12091101]

@bradenmacdonald, thank you for approving the PR! Could you please merge it as well?

[bradenmacdonald]

@Anas12091101 From what I've heard, we need to wait until after Sumac has been cut to merge it, because Sumac will still support Node 18 and 20.

[brian-smith-tcril]

Linking the DEPR for adding context openedx/public-engineering#280