docs: release notes for the v21.0.0-rc.2 release #2397

Job	Run time
list	25s
workflow (build-cli)	1m 47s
workflow (modules-builder-tests)	1m 54s
workflow (devkit-core-tests)	1m 26s
workflow (build-unit-tests)	2m 31s
workflow (devkit-core-tests-rerun)	1m 32s
	9m 35s

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