docs: release notes for the v21.0.0-rc.0 release #1173

Job	Run time
list	22s
workflow (devkit-core-tests-rerun)	24s
workflow (modules-builder-tests)	26s
workflow (build-cli)	28s
workflow (devkit-core-tests)	28s
workflow (build-unit-tests)	25s
	2m 33s

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