submodule: bump third_party/openthread/repo from `521f95f` to `fd2e06d` #6845

Job	Run time
buildx (latest, ubuntu-24.04, ubuntu:bionic, linux/amd64, yes)	2m 23s
buildx (latest, ubuntu-24.04-arm, ubuntu:bionic, linux/arm/v7,linux/arm64, yes)	3m 33s
docker-check	4m 9s
buildx (focal, ubuntu-24.04, ubuntu:focal, linux/amd64, yes)	2m 53s
buildx (focal, ubuntu-24.04-arm, ubuntu:focal, linux/arm64, yes)	4m 22s
buildx (reference-device, ubuntu-24.04, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg...	2m 19s
buildx (reference-device, ubuntu-24.04-arm, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build...	3m 16s
buildx (test, ubuntu-24.04, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEV...	2m 20s
buildx (test, ubuntu-24.04-arm, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE...	3m 59s
	29m 14s

Provide feedback