submodule: bump openthread from 4c0af02 to c8fa6e3 (#930)

Bumps [openthread](https://github.com/openthread/openthread) from `4c0af02` to `c8fa6e3`.
- [Commits](openthread/openthread@4c0af02...c8fa6e3)

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

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