submodule: bump openthread from 3ebc3d3 to 9d6321b (#705)

Bumps [openthread](https://github.com/openthread/openthread) from `3ebc3d3` to `9d6321b`.
- [Commits](openthread/openthread@3ebc3d3...9d6321b)

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

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