submodule: bump third_party/openthread/repo from `a0f861d` to `7b72e97` #6688

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	31m 47s
docker-check	4m 32s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	22m 10s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	20m 43s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	27m 38s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	32m 13s
	2h 19m 3s

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