submodule: bump third_party/openthread/repo from `84e80c6` to `f6cee79` #6767

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	33m 21s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	23m 2s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	21m 28s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	28m 48s
docker-check	4m 46s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	33m 8s
	2h 24m 33s

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