submodule: bump openthread from c10f650 to 6009dec (#578)

Bumps [openthread](https://github.com/openthread/openthread) from `c10f650` to `6009dec`.
- [Commits](openthread/openthread@c10f650...6009dec)

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

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