submodule: bump openthread from 4a0d6b7 to 6bc3b4d (#737)

Bumps [openthread](https://github.com/openthread/openthread) from `4a0d6b7` to `6bc3b4d`.
- [Commits](openthread/openthread@4a0d6b7...6bc3b4d)

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

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