docs: release notes for the v20.3.21 release #2877

Job	Run time
build-e2e-windows	28s
build	31s
lint	2m 0s
e2e-windows	-1s
Browser Compatibility Tests	-1s
publish-snapshots	-1s
test	-1s
e2e	0s
e2e-snapshots	0s
e2e-package-managers	0s
	2m 55s

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