docs: release notes for the v21.0.4 release #2766

Job	Run time
list	22s
workflow (devkit-core-tests)	1m 29s
workflow (devkit-core-tests-rerun)	1m 33s
workflow (modules-builder-tests)	1m 42s
workflow (build-cli)	1m 51s
workflow (build-unit-tests)	2m 33s
	9m 30s

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