submodule: bump openthread from 819938d to b3c4a7e (#927)

Bumps [openthread](https://github.com/openthread/openthread) from `819938d` to `b3c4a7e`.
- [Commits](openthread/openthread@819938d...b3c4a7e)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: b3c4a7e58331e5720d3ce0c2baddaaa43b43fcbe
  dependency-type: direct:production
...

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