submodule: bump openthread from a8632ac to 9c21165 (#874)

Bumps [openthread](https://github.com/openthread/openthread) from `a8632ac` to `9c21165`.
- [Commits](openthread/openthread@a8632ac...9c21165)

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

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