test: Remove support for Node 18

[arslanashraf7]

Description copied over from #352

This PR fixes the conflicts in #352.

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 #319 for further information.

[openedx-webhooks]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c9d0abe) to head (3e8b6ab).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #365   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          111       111           
  Lines         1088      1088           
  Branches       165       165           
=========================================
  Hits          1088      1088           

☔ View full report in Codecov by Sentry.
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[arslanashraf7]

@brian-smith-tcril Thanks for the review. I don't have the merge rights so feel free to merge it whenever you would like.

[brian-smith-tcril]

@arslanashraf7 branch protection rules on this repo require branches be up to date before merge. I just merged another PR that had been waiting so this needs a rebase now. I've already communicated my plan to merge this PR, so as soon as it is rebased I'll hit the button!

[arslanashraf7]

@arslanashraf7 branch protection rules on this repo require branches be up to date before merge. I just merged another PR that had been waiting so this needs a rebase now. I've already communicated my plan to merge this PR, so as soon as it is rebased I'll hit the button!

@brian-smith-tcril rebased.