submodule: bump third_party/openthread/repo from `8d39758` to `7f6723f` #6629

Job	Run time
docker-check	4m 3s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	31m 57s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	22m 22s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	21m 16s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	29m 15s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	32m 57s
	2h 21m 50s

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