Bump NodeJS to version 20

[RichDom2185]

Node 16 is EOL and Node 18 has build problems, but Node 20 seems to work fine and all tests pass locally on my MacBook.

TODOs:

• [x] Update CI
• [x] Update README

[coveralls]

Pull Request Test Coverage Report for Build 7593133001

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 82.937%

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Totals
Change from base Build 6702227868:	0.08%
Covered Lines:	10893
Relevant Lines:	12706

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💛 - Coveralls

[RichDom2185]

Looks good! Let's have this merged and ready ASAP so that it becomes easier to setup with the same node version as the frontend.

Great, thanks! I've updated (and reformatted) the README as well, will merge after the CI passes.