submodule: bump openthread from 8b906eb to 51ab865 (#491)

Bumps [openthread](https://github.com/openthread/openthread) from `8b906eb` to `51ab865`.
- [Commits](openthread/openthread@8b906eb...51ab865)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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