docs: release notes for the v21.2.6 release #3360

Job	Run time
list	23s
workflow (devkit-core-tests)	1m 36s
workflow (build-unit-tests)	2m 32s
workflow (devkit-core-tests-rerun)	1m 33s
workflow (build-cli)	1m 45s
workflow (modules-builder-tests)	2m 0s
	9m 49s

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