docs: release notes for the v21.2.9 release #3007

Job	Run time
build	28s
lint	1m 51s
build-e2e-windows	29s
test	0s
publish-snapshots	0s
Browser Compatibility Tests	0s
e2e-snapshots	0s
e2e-package-managers	0s
e2e	0s
e2e-windows	-1s
	2m 47s

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