chore: add Node.js 21 to `test-module` workflow #655

Job	Run time
Lint using ESLint	27s
Test / prepare-node-matrix	5s
Test / test (18, ubuntu-latest)	2m 43s
Test / test (18, macos-latest)	2m 12s
Test / test (18, windows-latest)	6m 8s
Test / test (20, ubuntu-latest, experimental)	2m 34s
Test / test (20, macos-latest, experimental)	2m 18s
Test / test (20, windows-latest, experimental)	5m 31s
	21m 58s

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