docs: create v21 announcement article #1881

Job	Run time
premerge-build	2m 3s
premerge-integration-test (integration:ng21)	2m 36s
premerge-integration-test (test:types)	1m 32s
premerge-test (test)	2m 49s
premerge-test (lint)	1m 29s
premerge-bundlemon	1m 46s
	12m 15s

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