submodule: bump third_party/openthread/repo from `b28bb1d` to `71aed82` #5054

Job	Run time
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	27m 40s
docker-check	3m 51s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	18m 23s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	17m 31s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	23m 53s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	27m 43s
	1h 59m 1s

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