chore: Update supported Node versions in README #506

Job	Run time
matrix	1s
build (ubuntu-latest, 18.x, stable)	2m 5s
build (ubuntu-latest, 18.x, nightly)	2m 1s
build (ubuntu-latest, 20.x, stable)	2m 11s
build (ubuntu-latest, 20.x, nightly)	1m 53s
build (windows-latest, 18.x, stable)	5m 34s
build (windows-latest, 18.x, nightly)	5m 21s
build (windows-latest, 20.x, stable)	5m 55s
build (windows-latest, 20.x, nightly)	5m 55s
build (macos-latest, 18.x, stable)	2m 19s
build (macos-latest, 18.x, nightly)	2m 50s
build (macos-latest, 20.x, stable)	1m 46s
build (macos-latest, 20.x, nightly)	2m 43s
	40m 34s

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