submodule: bump third_party/openthread/repo from `cc8f078` to `3702c88` #6600

Job	Run time
docker-check	4m 2s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	30m 52s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	21m 50s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	20m 33s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	28m 20s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	32m 12s
	2h 17m 49s

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