docs: release notes for the v21.2.4 release #3310

Job	Run time
list	21s
workflow (devkit-core-tests-rerun)	1m 20s
workflow (devkit-core-tests)	1m 39s
workflow (build-cli)	1m 42s
workflow (build-unit-tests)	2m 17s
workflow (modules-builder-tests)	1m 40s
	8m 59s

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