docs: release notes for the v21.2.16 release #2098

Job	Run time
list	27s
workflow (build-cli)	41s
workflow (devkit-core-tests-rerun)	37s
workflow (devkit-core-tests)	33s
workflow (build-unit-tests)	33s
workflow (modules-builder-tests)	29s
	3m 20s

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