docs: release notes for the v21.1.0-rc.0 release #2830

Job	Run time
list	23s
workflow (devkit-core-tests-rerun)	1m 31s
workflow (build-cli)	1m 33s
workflow (build-unit-tests)	2m 38s
workflow (modules-builder-tests)	1m 55s
workflow (devkit-core-tests)	1m 15s
	9m 15s

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