submodule: bump openthread from afc55a3 to adf4e84 (#256)

Bumps [openthread](https://github.com/openthread/openthread) from `afc55a3` to `adf4e84`.
- [Commits](openthread/openthread@afc55a3...adf4e84)

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

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