docs: release notes for the v21.2.15 release #3745

Job	Run time
list	31s
workflow (build-unit-tests)	2m 37s
workflow (devkit-core-tests)	1m 48s
workflow (devkit-core-tests-rerun)	1m 52s
workflow (modules-builder-tests)	2m 25s
workflow (build-cli)	1m 59s
	11m 12s

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