submodule: bump openthread from 6694d5c to a8632ac (#873)

Bumps [openthread](https://github.com/openthread/openthread) from `6694d5c` to `a8632ac`.
- [Commits](openthread/openthread@6694d5c...a8632ac)

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

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