Drop package manager CI for yarn and pnpm, add Node v20 to CI

[coveralls]

Pull Request Test Coverage Report for Build 8813382465

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.0%) to 48.052%

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Totals
Change from base Build 8786221815:	2.0%
Covered Lines:	195
Relevant Lines:	367

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💛 - Coveralls

[mcollina]

I currently have no time to figure what's making pnpm unhappy, and removing it seems the quickest solution to our red CI.