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 89.23%. Comparing base (64f92de) to head (1d27867).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1454   +/-   ##
=======================================
  Coverage   89.23%   89.23%           
=======================================
  Files         315      315           
  Lines        5545     5545           
  Branches     1372     1372           
=======================================
  Hits         4948     4948           
- Misses        581      582    +1     
+ Partials       16       15    -1     

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[bradenmacdonald]

Note: per openedx/public-engineering#280 we can only merge this after Sumac is cut.

[bradenmacdonald]

@BilalQamar95 Note: before merging this, we need to update it to not use matrix for the coverage upload name.

frontend-app-learning/.github/workflows/validate.yml

Lines 24 to 26 in d47433e

	name: code-coverage-report-${{ matrix.node }}
	# When we're only using Node 20, replace the line above with the following:
	# name: code-coverage-report

frontend-app-learning/.github/workflows/validate.yml

Lines 36 to 38 in d47433e

	name: code-coverage-report-20
	# When we're only using Node 20, replace the line above with the following:
	# name: code-coverage-report