docs: release notes for the v21.2.19 release #3906

Job	Run time
list	25s
workflow (devkit-core-tests)	1m 32s
workflow (build-cli)	2m 24s
workflow (modules-builder-tests)	2m 47s
workflow (build-unit-tests)	2m 58s
workflow (devkit-core-tests-rerun)	1m 42s
	11m 48s

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