submodule: bump third_party/openthread/repo from `68a5ce3` to `15a7892` #6883

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	35m 23s
docker-check	4m 42s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	23m 31s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	31m 58s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	36m 13s
	2h 11m 47s

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