docs: release notes for the v21.0.0 release #2475

Job	Run time
list	19s
workflow (devkit-core-tests)	1m 17s
workflow (modules-builder-tests)	1m 48s
workflow (devkit-core-tests-rerun)	1m 28s
workflow (build-unit-tests)	2m 14s
workflow (build-cli)	1m 41s
	8m 47s

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