docs: release notes for the v21.2.10 release #1964

Job	Run time
list	18s
workflow (modules-builder-tests)	34s
workflow (build-cli)	20s
workflow (devkit-core-tests)	23s
workflow (build-unit-tests)	22s
workflow (devkit-core-tests-rerun)	23s
	2m 20s

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