docs: release notes for the v21.2.0-rc.0 release #3021

Job	Run time
list	29s
workflow (devkit-core-tests)	1m 51s
workflow (build-cli)	2m 8s
workflow (devkit-core-tests-rerun)	1m 37s
workflow (build-unit-tests)	3m 0s
workflow (modules-builder-tests)	2m 17s
	11m 22s

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