submodule: bump openthread from 0cf450a to 8fbe09e (#1151)

Bumps [openthread](https://github.com/openthread/openthread) from `0cf450a` to `8fbe09e`.
- [Commits](openthread/openthread@0cf450a...8fbe09e)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 8fbe09e2b5a616f43ed8a221a1bce034e8b7fdb4
  dependency-type: direct:production
...

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