feat!: bump to Node 20

[alcpereira]

Solves #768

I've done the following:

• update the .nvmrc
• update link for npm docs to the version used by Node 20

Some open questions:

• I've added ! and BREAKING CHANGE in the commit, let me know if you agree that it should be a breaking change, it seems more like a internal change rather than a real breaking change for end users (for example we do not specify engines in the package.json)
• I've also updated the workflow to run tests only on Node 20, not sure if we should still test old versions

Everything seems to be working as expected.

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