chore: simplify setup-node gh action use

[brian-smith-tcril]

NODE_VER doesn't seem to be used anywhere else in the repo, so I figure it makes sense to utilize the setup-node action's built in node-version-file functionality

excerpt from CI on this PR

Run actions/setup-node@v3
  
Resolved .nvmrc as 18
Found in cache @ /opt/hostedtoolcache/node/18.1[7](https://github.com/openedx/frontend-app-learning/actions/runs/6188199250/job/16799694086?pr=1185#step:3:7).1/x64

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.86%. Comparing base (f58d6d6) to head (e0870c3).
Report is 171 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1185   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files         263      263           
  Lines        4483     4483           
  Branches     1133     1133           
=======================================
  Hits         3939     3939           
  Misses        530      530           
  Partials       14       14           

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[bradenmacdonald]

Looks good but closing in favor of #1454 which will make the same change. We can merge it after Sumac is cut.