docs: release notes for the v20.3.21 release #1807

Job	Run time
list	30s
workflow (devkit-core-tests)	24s
workflow (modules-builder-tests)	25s
workflow (build-cli)	22s
workflow (devkit-core-tests-rerun)	31s
workflow (build-unit-tests)	23s
	2m 35s

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