submodule: bump openthread from 7d488bc to 4c0af02 (#929)

Bumps [openthread](https://github.com/openthread/openthread) from `7d488bc` to `4c0af02`.
- [Commits](openthread/openthread@7d488bc...4c0af02)

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

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