Node 20 upgrade

[arbrandes]

Description

See the parent ticket for the general description and motivation, openedx/public-engineering#267. Make sure to follow its recommendations for each step of the process.

Tasks

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1. test: Add Node 20 to CI matrix #1069
   +0
   
2. build: Upgrade to Node 20 #1056
   +0
   
3. test: Remove support for Node 18 #1075
   waiting for eng review +1