docs: release notes for the v21.2.0-rc.1 release #3054

Job	Run time
list	26s
workflow (modules-builder-tests)	2m 6s
workflow (build-cli)	1m 39s
workflow (build-unit-tests)	2m 29s
workflow (devkit-core-tests)	1m 21s
workflow (devkit-core-tests-rerun)	1m 36s
	9m 37s

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