submodule: bump third_party/openthread/repo from `266a97b` to `ac7f75… #621

Job	Run time
docker-check	3m 54s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	27m 48s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	19m 26s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	17m 26s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	24m 20s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	28m 10s
	2h 1m 4s

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