submodule: bump third_party/openthread/repo from `9106817` to `91b7c3f` #4853

Job	Run time
docker-check	5m 41s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	53m 19s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	25m 38s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	27m 26s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	42m 44s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	56m 37s
	3h 31m 25s

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