submodule: bump third_party/openthread/repo from `ac7f751` to `8243591` #4991

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	26m 46s
docker-check	4m 29s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	18m 27s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	17m 45s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	23m 59s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	27m 59s
	1h 59m 25s

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